Most likely my 4th Ed traveller books. Played way back in the uni and unlikely to run traveller.
Mmm. The good old days. You know that campaign was nearly 30 years ago? Yes, it seems unlikely that my T4 stuff will ever see the light of day again either, unless you count that adventure that I'm still schlepping around conventions. I still need to write the sequel that was supposed to be ready for the 1998 con season...
Likewise my classic and MegaTraveller collection will stay boxed up, except for reference duties. MgT2 will be my Traveller for the moment.
I think my BECMI D&D collection is also likely to stay in the attic, because I ran a 3-year campaign back in the 00's and the group haven't forgotten yet. One still uses his Rules Cyclopedia as his default book to store/lean a character sheet on.
Hero Wars is going to stay boxed, wherever it is, because if I want to go back I have HeroQuest Glorantha now.
Golden Heroes. I have a couple of copies in the attic, got a decent amount of play out of it in the old days. Terrifying to think that it was a bit of a nostalgia trip when we played it back in the mid 90s!
Shadowrun. I only have 1st ed, because I'd already gone off it by the time 2nd ed was released.
Vampire the Masquerade. I have 1st and 2nd ed of this, although 1st ed has been punched and put in a binder because the spine disintegrated within a year or two. I was done with it well before the turn of the millennium.
Right, I think that's just about everything I seriously played before the age of 30 consigned to history, with the exception of WEG Star Wars which will always be the best Star Wars RPG and I played as recently as last year.