Google is literally the only manufacturer that lets you unlock your phone day 1, officially, without any "e-fuses" or warranty voiding, with step by step instructions on their own website, and you're saying they are killing custom ROMs?
Perhaps custom ROMs are dying because people don't feel a need to flash them? I haven't flashed a custom ROM in probably 8ish years, because my phones stock ROM has everything I want. I no longer need to flash a ROM for flashlight toggle, screen recording, scrolling screenshots, etc. It's all built into my OEM ROM.