For what it's worth: “All Chromium extensions are supported and work right away, by default, including all MV2 extensions. We'll keep support for MV2 extensions for as long as possible.”

Whether that's worth much is of course another matter.

Do Helium, or any other browsers with intent to keep supporting MV2 extensions say what they intend to use as a repository to acquire them?

I don't think the chrome or microsoft extension websites even let you upload a MV2 extension anymore, and most chromium forks I've used rely entirely on the chrome web store.