My stock ROM stopped getting updates in school, so I started flashing custom ones just to keep up. That turned into two years as an official maintainer, building and shipping releases from source for a device the manufacturer had moved on from.
My phone, a Redmi S2/Y2 on the msm8953 chipset, stopped getting updates almost as soon as I owned it. That is an ordinary complaint. What made it stick was that flashing a custom ROM to fix it turned out to be more interesting than the phone itself: the whole build, AOSP source down to the kernel, was open, and nobody was shipping a build for this specific device that felt current.
Flashing led to building, and building led to being asked to maintain. I ended up an official maintainer on two ROM projects for the same device at once.
Custom ROMs as a hobby are basically dead. Manufacturers ship longer support windows now, bootloaders lock down harder, and the phones worth flashing stopped being worth flashing. I do not think that is a loss to mourn, it is just what happens when the problem that pulled me in gets solved a different way.
What it left me with was not the ROM. It was being comfortable reading kernel source, debugging a build that fails with no stack trace to point at, and shipping something a few thousand strangers actually installed, before I had written a line of production web code.