So it appears I misspoke, the Endless shell doesn’t currently ship with any extensions, so there’s not even a whitelist. With the 3.2 release we made a big jump forward from GNOME 3.18 to 3.22, which “in theory” will make it easier to support extensions in the future, but for the time being it’s not something we enable in Endless
Yeah, as @Thomas_Sattler says we do have some extensions which were added by the Endless Coding project late last year although it’s not guaranteed they will work at all, because since then we’ve moved forwards about 3 years in shell development as you said.
However, we don’t have any white/blacklist or do anything explicitly to disable them, but the thing we lack is a UI to install/enable them. The best thing I think would be a Flatpak of GNOME Tweak Tool, which would meet this and some other needs. Otherwise I guess you can unpack stuff into ~/.local/gnome-shell/extensions/ but I don’t know how @Thomas_Sattler was enabling them…