I believe people should be able to modify the program to suit their needs, and I picked a license that allows them to do that. There’s quite a difference between adding features to a personal fork, and the nuclear option of creating a separate distribution and separate iOS client however.
AnkiMobile is not open source, because a) it required a complete rewrite for a proprietary OS at the time, and b) it would be much harder to raise any funds if anyone could trivially build their own version.
The only plugin system that Apple would allow on iOS is a JS one. Anki has been gradually migrating its old Python-centric code to JS, but there is still a lot of work to do, and we have had little time so far to flesh out how JS plugins should work. If you wish to participate in the discussion or implementation, @Rumo created this ticket a few days ago: How to make web pages extensible by add-ons? · Issue #3187 · ankitects/anki · GitHub