Honestly I don't see much benefit in adding libravatar service as was originally supported - the federated avatar idea never really worked and most people who originally set it up forgot and it did lead to invalid URLs (I've seen lot of them even with quite small user base we have on Hosted Weblate).
Supporting Libravatar server is just mater of changing AVATAR_URL_PREFIX and there is no need for special support for this.
But at least AVATAR_URL_PREFIX should be documented, what it currently is not.
Thank you for your report, the issue you have reported has just been fixed.
@nijel https://www.libravatar.org/ is up and running from Fedora, which seems a tad better than specifically leaking info that the e-mail is registered on yet another service(?)
Source-code is crucially available here.
Please see https://github.com/WeblateOrg/weblate/issues/3564#issuecomment-595649477 for proposed solution