Quick search of
https://sources.debian.org/src/cups/2.3.0-7/backend/ipp.c/ shows there
is no different between ipp and http, and no difference between ipps and
https.
ipps and https force encryption, using SSL/TLS (just like you'd expect
from https) (so if your printer doesn't offer encryption, say due to no
certificates, it'll fail).
ipp and http do not require encryption (but may use it anyway, if
available, via a TLS upgrade).
Probably CUPS offers both because some printers' documentation gives
http:// URLs and some give ipp:// URLs.
