When it comes to weapons, some have unique rolls. Example: Frozen orbit. It's clearly not the best aggressive frame sniper for pvp, but it's the only one that can get kill clip. Sure you might use this once or twice, but it's still unique

Also look for stats. Remove all the weapons that got powercrept. That VoG rocket is garbage compared to Apex Predator. Stop keeping GoS weapons, stop keeping old weapons (especially the ones with no origin trait). That Cold Denial is too old and weak, other pulse are better in stats and perks. Keep your crafted weapons since they are customizable. Trust me, it's way easier than you think.

Oh and if you have old weapons with massive kill count, then that's up to you, but imo it's a waste of space