I was surprised
A JMAP-supporting email host for the domain example.com SHOULD publish a SRV record _jmaps._tcp.example.com which gives a hostname and port (usually port 443).
The authentication URL is https://hostname/.well-known/jmap (following any redirects).
Other autodiscovery options using autoconfig.example.com or autodiscover.example.com may be added to a future version of JMAP to support clients which can’t use SRV lookup.
It doesn't match the original use cases for the well-known URI registry. Stuff like robots.txt, or dnt / dnt-policy.txt. And IPP / CUPS printing works fine without it, using a DNS TXT record to specify a URL. If you can look up SRV records, you can equally look up TXT. And the autodiscovery protocol involves XML which can obviously include a full URI.
E.g. what chance is there of this being accepted by the registry of well-known URIs? Or is it more likely to remain as something non-standard, like made-up URI schemes?
The idea almost certainly came from CalDav, which is already in the registry of well-known URIs. RFC 6532 defines DNS SRV and both DNS TXT and a well-known URI. So JMAP's proposal is perfectly well-founded.
It might sound strange that the URL is authenticated against, but this too is justified in CalDav. I think it helps shard users between multiple servers.
IMO it's not a good way to use SRV. On the other hand, JMAP is specifically considering clients that don't use SRV. One presumes the CalDav usage is for similar reasons.
It does seem bizarre that presumably web-centric implementations can't manage to discover full URIs (i.e. if they're using the autoconfig protocol).
I think you have to remember that these approaches start from user email addresses. The hallowed Web Architecture using HTTP URIs for everything... well, let's say it doesn't have much to say about mailto: URIs. DNS has got to be the "right" way to bridge the gap from domains to URIs. But in a web-centric world where you don't necessarily know how to resolve DNS, or only how to look up IPs to speak HTTP with? There are going to be some compromises.