Hey!
interesting thread.
I migrated a 2007 phpBB with 300000 posts to Discourse about 2 weeks ago.
The website URL changed from http://www.vw-camper.fr to https://vw-camper.fr with a proper redirect from www to no subdomain.
Also, the migration script set proper 301 redirections from old links to new links.
Multiple users complained that typing “vw camper” in Google doesn’t display the site in top results anymore, even not on the first page.
Also, a weird thing… Not very important, just being curious. This is what one of my users reported when typing “vw camper.fr” related string on google:
This is the old website description but… What is this hideous URL?
http://www.vw-camper.frwww.vw-camper.fr 
Just a bug I believe?
Is that still valid information after a migration in 2022? Should we just wait a few months for the forum to regain its previous SEO ranking?