the Steam version has achievements, so why doesn't this version?

Give them time, Galaxy is still beta. They may get them in eventually.

Yes, once it reaches a final state in like 6 months, they'll surely start to add more and more things like this.

Because this version was published before anyone had even heard about Galaxy.

And does the Galaxy even support achievements? (I have no idea, because I'm still using the old GOG Downloader and I have no intention to update it to Galaxy anytime soon).

it does. GOG stated that all games that had existing achievement support would get updated in Galaxy. just hasn't happened for some games yet, I guess

Any idea if it would have it, eventually?

It's the year 2022 and no Achievements for the game on GoG. I don't think they are going to do it, for whatever reason.