I have an offline backup installer of Ion Fury 1.0 from the day the game was released on GOG. Back then the EULA of GOG stated that all previous versions of games will be kept available for download even if new updates are issued, since many users have different configurations and require different versions for differen things. But I can't find the 1.0 download anywhere on GOG or GOG Galaxy. I know it existed because I literally have it on my PC.

This is not the case for any game on GOG. Generally, only Galaxy can revert to the penultimate build if it hasn't been manually removed (as is the case with Ion Fury v1.1).

Well, it used to be so that previous versions were maintained but it obviously changed. How does one find the previous versions of a game on Galaxy? I'm not finding any function on Galaxy that allows the downloading of older version installers. Also, why would previous installers be removed anyway? As you said, Ion Fury 1.1 is gone, but so is 1.0. For many players, old versions are necessary because updates can break mods or speedrun strategies, etc.

Our join dates are quite close yet I don't recall it ever being the case. Right-click on a game, then go to Configure, and from there, disable the option to keep the game automatically up-to-date. It'll give you options to force versions 2.0 or 1.1. I think it's just an automated practice.

Hmm... I added my offline-installed Ion Fury to Galaxy, and disabling the auto-update does indeed recognize the 1.0 install, but I can't confirm whether it's just showing the version I have installed, or if it's actually offering that version for download. I'd need to uninstall the game to see if it's possible to download the release version. 1.02 and 1.1 are available, so it would make sense that 1.0 is too. If it isn't then there's something fishy going on, because it would need to have been manually deleted while the other versions were preserved.