Yes, a ton of items are no longer available - literally dozens. 4 or 5 months ago, the game had a massive update (thus: supermechs reloaded) and all (or at least most) items from before the update are considered ‘legacy’ and no longer obtainable.
I should note that some items also only appear at legendary rarity and above; naturally, they are quite rare, but are still obtainable.
As for a list of items available, I’m not aware of a complete one - but a list of all items that go to mythical rarity is available on this forum right here. These are really the only ones you need to worry about, honestly.