The download page suggests getting the Nextcloud PGP key using this command:

gpg --keyserver ha.pool.sks-keyservers.net --recv-keys 28806A878AE423A28372792ED75899B9A724937A

The SKS-Keyserver network is going down. https://sks-keyservers.net/ has the following statement (if you ignore the certificate warning):

This service is deprecated. This means it is no longer maintained, and new HKPS certificates will not be issued. Service reliability should not be expected.
Update 2021-06-21: Due to even more GDPR takedown requests, the DNS records for the pool will no longer be provided at all.

And indeed, ha.pool.sks-keyservers.net no longer resolves, breaking the signature verification instructions.
What is the preferred alternative now? key.openpgp.org? The key seems to be available there, but the identity is not verified.

Of course is is also still available on https://nextcloud.com/nextcloud.asc.
However, I personally would prefer some standardized solution that is directly supported by gpg. Aside from keys.openpgp.org, Web Key Directory (WKD) comes to mind. In a nutshell, WKD only requires hosting the key in a well-known location on nextcloud.com (see https://wiki.gnupg.org/WKD and https://wiki.gnupg.org/WKDHosting).

In summary I'd like to suggest doing at least one (but preferably both) of the following:

• verify the key's identity on keys.openpgp.org
• set up a WKD for the key

And, of course, the signature verification instructions should be changed accordingly.