Perhaps you mistook the two bullet points outlining what currently happens as goals for the standard?
If the email address isn't yet known to this third party (or, you are not logged in), there _will_ be a context switch which in my example case will occur for every registration since I use a per-entity email address.
<my initials>-site-<companyname>@<my domain> go to my personal mailbox
<my partner's initials>-app-<appname>@<her domain> go to my partner's mailbox
<daughter initials>-account-<entity name>@<my domain> go to my daughter's account
Sure you could in theory set up the server side verification mecanism for these pattern too. I am just stating that the +suffix stuff is not the only way used.