The Steam version of the game got an update to version 1.9 and even 1.9b a while ago, while we're still stuck at 1.8.9. Will we get the update as well? And if so, is there an ETA?

Meanwhile the Steam version is 1.09c (dated 25 Feb 2020), with many changes and bug-fixes. When is GOG giving its customers the latest version?
For now, I can only inform potential buyers that the current GOG version is more than THREE YEARS behind, while the Steam version is uptodate. During those 3 years a large number of bugs were fixed and new functionality was added.

If GOG doesn't bother to inform its paying customers about the reasons for the delay of more than 3 years in providing the uptodate version of the game, perhaps the developers would like to comment (assuming they have a look at the GOG forum of their game at all)?
Is there any connection with the following little note accompanying the release of 1.09c: "Without extra comment, I will also add that Interestellaria is no longer being published by Chucklefish."

As long as there is no meaningful explanation from GOG, I have to admit (grudgingly) that at least for this game Steam is providing better service to its paying customers than GOG, and this may influence future buying decisions ...

I just wrote him on twitter: https://twitter.com/redwarp/status/1257950103912042497
Let's hope he answers! (apparently, he is also not being published by Chucklefish anymore so...)

He answered me! https://twitter.com/ColdRice_Dev/status/1258144788877938688

Stay tuned!

nice one, thanks for getting in touch with the dev :)

has the game already been updated yet?

Not sure, I don't really want to buy it until it is, and there is no version visible on GoG.

Game is still on 1.8.9:
https://www.gogdb.org/product/1436951929#builds