Question: will old archive links remain working after the migration?
I mean if somebody in the past shared in some doc, article or in some ticket a link to a mailing list discussion.
That can even be a reference from one mailing list message to a thread in another mailing list or in the same mailing list.
Ideal would be to keep such links working.
Yes. I think it's best we leave the existing pages in place. They're static pages, so there's no load on the server to serve them whatsoever. I do think we need to add a header to all the pages saying that there are newer (searchable) archives elsewhere and then pointing to those archives. The pages which used to host the subscriptions should probably be auto-forwarded to the new subscription page.