Which keyservers can I use for
gpg --keyserver "$keyserver1" --recv-key keyid that I can expect not will go away anytime soon?
The recommendation is to use
keys.openpgp.org, however this keyserver only includes User IDs for keys whose owners have personally confirmed via email (basically eliminating large swaths of of the PGP ecosystem). It also does not include any 3rd party signatures on keys to mitigate the possibility of a "poisoned key" attack. As of December 2021, this is the default (if none is configured by the user) keyserver for GnuPG packaged by Debian since
gnupg2 2.2.17-1 (released in 2019).
Personally, I'd recommend a Hockeypuck-based keyserver like
keyserver.ubuntu.com, which isn't so limited (although it does strip 3rd party signatures). GnuPG has since changed this to the default as of versions 2.2.29 and 2.3.2.