I use Fennec, and it allows adding any Desktop extension (idontcareaboutcookies, privacyredirect etc.).

Only if you register with AMO.

It seems like Iceraven doesn't have this restriction, and works like Firefox on Android originally used to?

Iceraven still has a list of allowed extensions, but they added several hundred extensions to the default list rather than Mozilla's dozen or so.

(It also supports custom lists like Fennec and FF Nightly.)

> Iceraven still has a list of allowed extensions

where is that list?

(It's linked to from Iceravens readme: https://github.com/fork-maintainers/iceraven-browser )