Not if the terms are described in the contract.
But apparently, according to common distribution contract with GOG a publisher has rights to:
1) not update its game on GOG while updating it on other stores (read: Steam);
2) release crippled "DRM-free editions" (see "Armello");
3) remove OS compatibility at launch, while collecting money during pre-order times;
4) release DLCs only for one particular OS (read: Windows™) while the game is available for other OSes (see Pathfinder: Kingmaker);
5) this, i. e. remove the game availability on a whim "effective immediately" (I won't be surprised if GOG ends up being fined for unauthorized sells after 0.0000001 second it was informed about this "cease and desist" order :) );
6) probably, other things.

I'd say GOG is rather liberal with its "Customer-first approach" when it comes down to dealing with big-bucks publishers. But GOG compensates its frustration dealing with small indie developers: in that case GOG becomes a highly curated elitist store with high standards and plentiful demands. :)

That is correct, however I seem to recall that previously games were removed within a few minutes of a post (of course they might simply have posted later in the process).

The contract and whatever was in it is invalid the moment the owner of the product has changed.

In fact there is a clause forcing those devs to update the GOG version not later than 48 hours after steam. There has been a tweet about it somewhere. The other question however is - what happens if the devs don't meet this requirement? Some games have been delisted recently because of missing updates. So at least it seems as if GOG is trying to do something about this image they have.

Didn't they remove it because of this in the end?

I don't think that is true. If you are referring to compatibility with older OS it might still be the case that these are compatible - GOG just can't give you a guarantee. But when I read the next point I guess you are referring to Linux or Mac which would make my argument invalid.

Well, the wording is a bit confusing (might be intentional though - if you think about it a bit). As it has been said, "the process may take some time" which kind of is the opposite of "effective immediately".

Clauses don't mean much when they aren't enforced. They may have deleted some completely abandoned games but they're more than happy to get money selling unpatched games.

Agreed - that's why I wrote this "what happens if the devs don't meet this requirement? ". That is simply a question I don't know the answer for. Maybe in the past the answer simply was "nothing" maybe they changed their politics in this regard and are now beginning to delist games ... I don't know.

This guessing game is getting annoying and I would rather prefer for GOG to drop the silent treatment and TELL us HOW they want to improve such things. We cannot say... we cannot say ... we cannot say ... GOGs communication skills are ridiculous to say the least! THAT is the thing I really dislike here.

Agreed. For a company large enough to certainly have a communications team they suck at communications. There is simply no valid reason for them not releasing certain details people have asked for. There would be no contractual obligation they could possible have for posting a general patch policy and guidelines for example, or general curation guidelines to make these things people complain about a little more transparent. Neither of those things would conceivably break any vendor contracts, so long as it isn't vendor specific.

Not quite. The acquisition means getting everything: not only IP and actives but also contractual obligations, debts and other liability.
I don't see any effort on this field. For example, "This War of Mine" is still at its 4.0.1 version.
They removed "Armello" after user backlash. Initially GOG tried to cover developer's decision not to release DLC's with "DRM-free Edition" euphemism.
Yes, I refer to non-Windows versions. "The Coma: Recut" and "The 25th Ward: The Silver Case" are the examples. There can be more.
I think GOG actually outdid it this time. No task should be expected to be completed the very instant it was given.
So, I don't think they were informed one minute (or how much it took "chandra" to type the news) prior to this "effective immediately" post. Also, as it was worded, GOG didn't even ask for time for itself to process technical nuances and ended up "violating" its own voided contract. :)
Something doesn't sum up here, that's for sure.

Yes, the whole situation seems quite unusual. I would have expected that the games were removed more or less immediately after being removed on other storefronts, but they still remain several days after (even if we don't count the weekend). Also as I stated before, these "effective immediately" delistings usually happens immediately in the ordinary sense of the word.
Just a thought, but I wonder if it is related to the website "upgrade", as it seems very difficult for them to change anything (although The Long Dark was removed as scheduled).

"Since Telltale's closure, previous seasons of The Walking Dead have been unavailable from digital storefronts. However, Skybound noted that, following its recent arrangements, these will be restored for purchase as soon as possible. It did warn, though, that, "It's likely that previous seasons of the game may be unavailable to purchase for a few days as we transition"."

source https://www.eurogamer.net/articles/2018-11-19-work-has-resumed-on-telltales-final-season-of-the-walking-dead

no biggie

Good riddance.

Season 2 ended perfectly and happily for me. I don't know anything about Season 3 but I got the vibe it'd be a negatively-viewed transitional series.

Combined with my suspicion that none of the choices matter with each new season, I'm not fussed. Besides, my girlfriend has it on Xbox and if choices don't matter, neither does the presumably now non-existant save importing.

I had the first 2 but didn`t have season 3, was waiting as long as possible for a sale. However after the notice I snapped up 3.

Thankyou, GOG, for the timely warning in my email. I thought I`d missed it when it said, `Effective immediately!`

Guess now I`ll see how Clementine and the baby boy AJ is getting on.

p.s. Also those of you complaining about èffective immediately`yet it isn`t immediate, why are you whining? Stop looking a gift horse in the mouth. Jeez, you guys love to shoot a good thing down. I can imagine next time something like this happens you`ll be whining when GOG does immediately remove it so fast even an email won`t warn you in time!

Stupid.

I'm not really complaining about it (although it may look like I am), but wondering why the removal happens in this way. I think sending out an email about the removal is a significant improvement, but I can't help but wonder why the process takes so long (more than 16 hours), as they have had ample time to prepare the it (it hasn't exactly been a secret that the games were removed from other storefronts).

I think I got the first walking dead from GOG connect, so I won't loose much if the series games don't return here. But considering that the first Walking dead game is TellTale's biggest success, and it's what allowed them to secure all their future games, it must be a masterpiece of its genre. I would be pissed to not be able to play the series... one day. Though at this point, I have so many games, that it has turned to a matter of, if I'll be able to live long enough, to end up playing it.