On Debian Bookworm when running apt-key add.
NOTE: RSPAMD is only used as an example. This method should work for any deprecated install instructions for apt-key.
While executing:wget -O- https://rspamd.com/apt-stable/gpg.key | apt-key add -
The following is observed:
"Key is stored in legacy trusted.gpg keyring (/etc/apt/trusted.gpg), see the DEPRECATION section in apt-key(8) for details."