I'm mostly trying to clarify if the parent poster knew about something more than Webfinger, since his post mentions "DNS/http(s) services"; wasn't sure if that referred to Webfinger, or something else.
> How would/should/could a service be expected to take my email and determine what services it is associated with?
By following the OIDC standard, namely, using Webfinger discovery. I was mostly confused since, AFAIK, OIDC only mentions webfinger, not DNS, so I was curious if the parent to my post knew something I did not.
> as an end-user I don’t particularly want that to be possible anyway
I don't see why not. Note that this is discovering who the identity provider associated with an email is in an automated way — this is not discovering what services you log in to with that identity. That is, for "example@gmail.com", Webfinger would discover that Google is the identity provider. This is something a human can — roughly — already discern from the email anyways; this just provides an automated means. Moreover, it provides a means for the service wanting to log you in to then subsequently redirect you to; there are certain configuration parameters needed beyond just the email.
The idea is to make it simple for the average user: just enter your email.
> just because my email address is something@gmail.com, does that mean Google should be used as the OAuth provider?
Not necessarily, but I think the OIDC flow there is pretty good. One could imagine an implementation whereby GMail allowed you to specify an alternative IdP for your email, and then responded to with that when queried. This is more to do with Google's implementation. (Which, AFAIK, doesn't implement Webfinger at all, so it is kind of moot. And while I doubt they'd ever implement this sort of thing, the point is more that an IdP under your control could / that nothing in the spec prohibits it.)
> Maybe I use that same email address as an Apple ID and as a Microsoft ID as well.
I think OIDC supports this. The Webfinger query is not limited to a single result. (Though, given the paucity of webfinger implementations, I don't know how well UIs would handle this. But again, implementation vs. specification.) Even if that didn't work, you could always enter your IdP hostname directly into the RP, and it could either Webfinger that, or just see if that hostname is the IdP by attempting to pull the metadata from it.
(It is sad that most OIDC implementing RPs only give you a few hand-selected buttons leading to major IdPs to choose from.)