Yeah unfortunately that Custom ROM scene is dying out, I see that plenty of older phones are still supported (e.g. Galaxy Note 3, Samsung S5, kind of all Xiaomis) but newer phones are just a handful of models which its bootloader can be unlocked, and then some of these have support for Magisk and TWRP but sadly no custom ROMs.
Edit: still the ability to root these and unbloat them is better than nothing.