I personally had 3 games disappear out of my gog library - two I could retrace from looking through my purchases and only got them back after writing to the support. the third I still don't know what it was.
also, yes, the sole use I have for gog nowadays is to look at the section of newly released games. I don't follow any gaming websites, youtubers or anything. how else should I know what's being released?
The safest bet is to immediately download after purchase and catalog your GOG library on a hard drive. Update it every so often. If need be, duplicate on bluerays or something.
Let's look at one example:
1. At one point there was Interplay Fallout 1 available for sale.
2. Bethesda bought rights to Fallout (but not to Interplay logo).
3. Just to be dicks - Interplay and GOG started giving away copies of Fallout 1 for free - for the last month when they could legally do so.
4. Fallout 1 was removed from GOG store - but not people's libraries.
5. Then - Bethesda started selling 'their' version for 2$ on GOG - without Interplay logo.
6. People who had Interplay Fallout - probably didn't want the one with Bethesda logo. So it was renamed 'Fallout 1 Classic' and excluded from automatic upgrades.
7. Then - people started complaining that they've bought Bethesda one by mistake - and they already had Interplay Fallout 1 - so Bethesda one was added to libraries of everyone who had Interplay Fallout 1.
8. Then - Bethesda removed Fallout 1 from GOG in 2014.
9. After few years - Microsoft bought Bethesda and Fallout 1 was brought back to GOG.
10. And now people complain that newly bought Bethesda version does not have Fallout Bible ...
I would lose track who has a right to download what - so downloading offline installers is probably a good idea.