With unbound 1.16.2, using
url: https://some.IP.v.4/path/filename.stuff
I can observe that this doesn't work and that I am still obliged to include an allow-notify line in the config for the rpz zone.

This should work without specifying an allow-notify if the IP the NOTIFY comes from is the same IP as "some.IP.v.4".
Also the qname of the NOTIFY needs to match the configured rpz zone name (I believe that is not your problem).
The testcase for this issue still works btw.

With verbosity 2 and above you would see something like the following in the log that may shed more light:

info: received NOTIFY for rpz. from 127.0.0.1 port 35684
info: refused NOTIFY for . from 127.0.0.1 port 34962

This is what I had in the config: url: https://a.b.c.d/mypath/myfile
Received a REJECT for the notify I sent from the same IP a.b.c.d.
Added an allow-notify: no error and the file was downloaded.

Do you mean REFUSE instead of REJECT?
Also could you post the relative unbound log entry?