Honestly, Custom ROMs have been in decline of usage since few years. There are also Other ROMS like Pixel Experience, PixyOS, Havoc, evolutionX, PixelOS, Paranoid, Derpfest, CrDroid and lot more.
The reason for the drop is due to a combination of reasons like better OEM UI, unpublished Kernel code (Chinese OEMs, Mediatek), locked bootloaders and Safetynet issues.
I'm currently rocking CrDroid it has currently ~85K active devices (https://stats.crdroid.net/).
Mastodon is by the biggest contributor to Fediverse as a whole. Has been adopted by tons of Orgs like EU, W3C, Verge, Flipboard, etc.