VSCodium, aka VSCode without the Microsoft proprietary bits, uses https://open-vsx.org/ as the extension registry. The hard problem is convincing devs to cross-publish
If more people were asking for it, I would try again, but the time spent per user to set up a pipeline for each extension just doesn't make sense at the moment. I'd rather spend those nights adding features for the existing mainline users. Cynical perhaps, but there's no reward for making extensions, so there's a limited amount of time I can spend on it.
I'd like to see a hard fork for VSCodium, where the community could make UX/DX upgrades that VSCode's Microsoft-employed gatekeepers have summarily shut down time and time again (see: issues on the find/replace dialogs)