A long time ago, there was Anki 1. It used a file format called ".anki". Very different from today's Anki2's ".anki2" format. You can still find on the web some old decks, shared a decade ago, in this format. This add-on allow to import them.

You should probably restart anki once you're done importing data. Importing .anki data essentially starts a copy of anki 2.0 which itself starts a copy of anki 1.0; that's not things you want to keep running and potentially interfering with your normal study. I can't guarantee there won't be any interference (especially in hooks)

The only new code is in aqt/importing.py, where I change the "onImport" function in order to deal with ".anki" file. If importing such a file is requested, it will import anki/importing/__init__.py, which itself will add all the required codebase. It essentially is a copy of the backend code and anki1 code, as it was before this feature was removed in all https://github.com/ankitects/anki/commit/1dce3eaaff649e2c1dbfd53bfe289cf42971045e#diff-c60e37732e601ec3d7f7e7146624b3a1 .

My best guess is that anki 1 was in python 2. Dropping the importer ensured that the code didn't have to be porter to python 3. I'm not certain, because tools exists to do the port.

Anyway, to get code from anki 1, anki 2.0 did used anki1 code to compute all cards. It actually simply opened this file as a collection and then used the function which ports collection from 1 to 2.0. I believe that there was no difference between decks and collection in anki 1; which might explain why it used anki 2.0 can import a deck and consider it as a collection.

Remove all part of the code not actually useful for anki