One thing I've embraced in my quest to slim down from "Hoarder" collector to "Minimalist" free range "person who can move easily" is the important philosophy that despite having a lot of general interest in the memory of the past, I actually don't have as much nostalgia for it as I had previously thought. This has helped a lot in freeing up stuff I have dragged around with me which I have held on to for no other reason than minor sentiment.
Another change in mindset has been to really think hard about the "stuff I will use and enjoy, and have used and enjoyed and will continue to do so," vs. "All the other stuff. Period." There are things I have (for D&D, for example) which get pulled out every week and will of necessity see use at some point, again and again. Then there are hundreds of other books I have because I was curious but they have never (and will never) actually see use. They are either redundant, not quite to my style, or distinct for their particular moment in the sun but not otherwise sustainable in the gaming ecology I swim in. D&D 5E, for example, is a staple these days.....but almost every OSR book I have fits in to the latter category of products I just won't ever be able to use for various reasons.
In fact, right now, the only OSR book I continue to hold on to is White Star Galaxy Edition and a couple key source books (Between Star & Void and Tools of the Worldshapers). There are plenty of good books in the OSR universe, but of all of it, only these are the ones I both read and used, and plan to use again. The truth is, if I decide to run classic D&D again, I am grabbing up 1st or 2nd edition AD&D and doing with the real thing, not a retroclone. And truth is....my tastes run very differently now from AD&D.
Genesys Core, for example, is the main game of choice for the last month or so now. I'm running a homebrew campaign with it using the core rules and Realms of Terrinoth. I'll talk more about this later, but needless to say those two books fall firmly in the "keep" camp.
If you're interested in what I've decided I must keep so far, it looks sort of like this:
D&D 5E (official books, a small number of 3PP)
Starfinder (it's new and hot and I have two games going)
Genesys Core (also new and hot, but the die mechanic has gone from "concerning" to the most important part of the experience just like that)
Traveller (MGT2nd, because it's the D&D of space, a guaranteed go-to system)
Cypher System (I'm exploring the idea of it now that Genesys has hooked me on story-focused systems of similar nature)
Savage Worlds (core and certain sourcebooks)
GURPS (I am getting rid of almost nothing here; GURPS remains Old Reliable)
Call of Cthulhu 7th Edition (I love this edition)
Basic Roleplaying and Fantasy World (Go to old reliables)
In addition to those, I've been collecting some books lately I still wish to collect and read, even if I never actually run them (this is why I fail to achieve my minimalist goal): Conan RPG, Infinity RPG, Symbaroum and a few others like Awaken and Mindjammer are impossible ignore, being interesting and well designed games. Who knows, maybe I'll find time to actually run them!
If it's not on the above list, though, I've probably gotten rid of it. Except for the Pathfinder Pocket Rule editions, those are too cute to get rid of (and portable, too!) Likewise for The Dark Eye's portable editions.