scdaemon should return this information together with other information about the smart card or the key slot.

For PIV I have tested which slot supports which curves with trial and error and then hard-coded this list (see PIVCard::supportedAlgorithms()). But that's nothing that Kleopatra should have to figure out or hard-code.

This (old) task only concerns OpenPGP smart cards resp. the OpenPGP card app, right? Because for PIV ECC has always been offered since PIV is supported. And for other card apps we do not even support generating keys AFAIK.

Right, this is only for the OPENPGP cards. Meanwhile we have
a way to get information on the supported algorithms. For example:

> scd getattr KEY-ATTR-INFO
S KEY-ATTR-INFO OPENPGP.1 rsa2048
S KEY-ATTR-INFO OPENPGP.1 rsa3072
S KEY-ATTR-INFO OPENPGP.1 rsa4096
S KEY-ATTR-INFO OPENPGP.1 nistp256
S KEY-ATTR-INFO OPENPGP.1 nistp384
S KEY-ATTR-INFO OPENPGP.1 nistp521
S KEY-ATTR-INFO OPENPGP.1 brainpoolP256r1
S KEY-ATTR-INFO OPENPGP.1 brainpoolP384r1
S KEY-ATTR-INFO OPENPGP.1 brainpoolP512r1
S KEY-ATTR-INFO OPENPGP.2 rsa2048
[...]

This works with Zeitcontrol cards since version 3.4 but it is not
available in current Yubikeys. Thus for now I think it is better to
implement this directly in Kleopatra:

if KEY-ATTR-INFO succeeds
   allow all listed algorithms
else if (Vendor == Yubikey and version >= 5.2.3)
   Allow:
   rsa2048
   rsa3072
   rsa4096
   brainpoolP256r1
   brainpoolP384r1
   brainpoolP512r1
   curve25519
else
   Allow:
   rsa2048
   rsa3072
   rsa4096

Note that for "curve25519" is is required to give to the internal APIs
"ed25519" for OpenPGP.1 and OpenPGP.3 but "cv25519" for OpenPGP.2

If support for KEY-ATTR_INFO is too much work right now, if is also
possible to add this condition:

else if (Vendor == Zeitcontrol and version >= 3.4)
   Allow:
   rsa2048
   rsa3072
   rsa4096
   brainpoolP256r1
   brainpoolP384r1
   brainpoolP512r1