Custom ROMs still exist, but have become a lot weaker in destinction than they used to.

In their "golden years" OEM Android distributions were just bad and came with inexcusable bloatware and restrictions. The main charm of Custom ROMs back then used to be that they were relatively cleaner. But now, with most Android phones coming with hardware powerful enough to make any impact of bloatware negligible, not to mention Android (and OEM iterations) itself having been converged into leaner, more efficient designs, the relative utility offered by Custom ROMs is fading fast.