Once, games had one single version: The one which were ready at deadline, when the floppies or cd-roms got printed. But today, with games being in development after release, and GOG versions potentially lagging behind Steam versions, wouldn't it be rather nice to know the actual version number?

Games in general should opt to place that version number somewhere within the main interface or even in the readme text file, yes. Especially those which will update frequently or plans to.

Yep! But I was more thinking that it would be nice to see the version number on the sales page, before buying the game. (I was thinking about getting Dream which currently has a nice discount, and wondered when it was last updated.)

When was that exactly? In the 90s, games did get patches, even if infrequent. Not sure about the 80s, but my guess would be that it would happen during the late 80s at least.
I do recall seeing patches for "Sam and Max: Hit the Road" in a promotional CD from a PC magazine, and I'm sure others will recall patches for earlier games as well. It's just that it was a pain to get said patches, at least until the CD-Rom became more widespread and magazines started providing those.

Actually, I believe even the original Wizardry got updates (that game was released in, I think, 1981). One update got rid of the age penalty for using the utility to recover an "out" party. Of course, there are some bugs that persisted even into the final version of the game, like the one involving HAMAN/MAHAMAN and the famous Identify glitch.