You're right that you can't personally farm them, but you can still end up getting them from publics. The old relics are still operable. I have hundreds of old relics that I open as fodder, as do other regular players. And ofc trading. The surplus of old relics helps keep down trade prices because they're still trickling into the market, albeit slower.

A better example is prime accessories. These are real-money only, and can only be bought from Prime Access (when a Prime is released) or a Prime Resurgence (Prime temporarily back for sale a year or so later). Stuff like Octavia Prime's shawzin/sitar being $35 or gone for (effecively) forever.