Well it's certainly a good note to add!

I was just wondering about this! Congrats to Alexim for putting it together!

Don't know if Septerra Core is on here, but there was apparently a Digital Deluxe content including soundtrack added to Steam a few weeks ago. This will not come to GOG.
Looks like Enclave too. There was digital deluxe content added, if the response I got from GOG support carries across, then that one as well.

I guess the digital deluxe edition adds extra costs for the OST and co., which GOG might not want to do for old games?

Thank you very much, it took a long time but it was worth it!

Support just said that it was not being added. Most likely devs or publisher can't be bothered to serve paying customers.

We are talking about paid soundtrack DLCs? I once contacted Killmondaygames and asked them about the payable soundtrack for Fran Bow which is absent here as well. The devs told me that GOG back then rejected it because their customers did not want to pay for soundtracks. When I told them that GOG has changed a lot they were very surprised and told me that they will try again when their next game will release.

Maybe other devs also think that GOG would reject adding paid soundtracks? Maybe we just need to make the devs/publishers aware that GOG meanwhile has quite a lot of those as well? Maybe zhis is the reason why the devs/publishers for Septerra Core ( Eclave did not bother to bring the deluxe content to GOG?

The Enclave and Septerra Core add-ons are free of charge to Steam users.

The Deluxe content? Because that is not free on Steam, unless I'm missing something.

Whoops, looks like it is for sale. Since they were already in my library the prompt for the DLC was not showing up.

As for devs thinking GOG rejects paid soundtracks. Are there not many games on GOG that have paid soundtracks available?

I can only assume it has something to do with the fact that GOG used to add a bunch of goodies/extras for games, including soundtracks, which wouldn't be sold separately then.

Not sure how to search for OSTs (or any DLC) specifically so I can't really answer the question. I know some soundtracks are available, I know we have a bunch of games on the list whose soundtracks are missing - and more to come - and I know that there's often a lot of rights stuff involved when publishing soundtracks, which in addition to GOG being a smaller platform makes it seem "not worth it" to some.

Added
Sunrider Academy & Liberation Day
Tangledeep
Teslagrad
The Colonists
The Curious Expedition (seems to be missing Traditional Chinese)
The Fall
The Guest

I thought I already explained this but maybe not clear enough: In the past GOG did in fact reject such things since soundtracks were added for free and their userbase was used to not pay for it. So it's not surprising that GOG rejected payed soundtracks back then. However - as you said - GOG did change and in the meantime they added quite some payed soundtracks to the store. It might be however that some devs did not realize this and therefore did not approach GOG again.

Added
The Way
Timespinner
To The Moon
Tower of Guns
Virginia
(missing soundtracks)

Thea: The Awakening (missing Coffee for Coding DLC)
Toren (missing Mac version)