Hi, if you archive your games (independent from stores) locally on a hard disk drive, do you also attempt to save relevant data which you could need later like

system requirements and description for niche indie games (Groupees, Itch) which will likely vanish from the Internet in the not so long run; also applies to fan sites of games (e.g. most of the GTA 1 fan sites are gone, the Internet Archive still has some of them, but that still means some files are missing)

mods etc (often the hosters move on with their site and later stop renewing the hosting)

If yes, do you do that manually by copying and pasting text to a text file or manually download mods etc.?

Yes, i do.
Mods, Widescreen fix (WSGF page), EXTRAS (if available), other fix, walkthrough and a read me.txt with instructions of what the hell are those for all goes in one folder which goes in inside game installer folder.

Same as amrit9037 and started doing that after some fan patches and mods I really considered essential vanished from the places where I used to find them.

If low on HD space, I rather save those than the games themselves if from GOG (or other shops).