I checked here (Discourse), StackOverflow, and the issue tracker (Jira).
As always Andrea Aime and Ian Turton had already answered a similar enough question which lead me to a viable workaround that I had not thought of myself. (Namely, to use a version’s release page to download the HTML zip of the documentation; ex: GeoServer .)
However, I have not seen anybody reporting that docs-archive as unavailable. It has been at least a few days (honestly, maybe a week or two). The HTTPS gives a Let’s Encrypt certificate that expired July 8th, and the HTTP gives a GitHub 404.