rspamd systems should not install spamassassin
Installing a debian 10 system with the default rspamd (options ./ispc3-ai.sh --debug --lang=en --use-ftp-ports=40110-40210 --no-mailman --unattended-upgrades
) still installs SpamAssassin (and later disables the daemon), this isn't needed or used in rspamd systems. (You actually can configure rspamd to use spamassassin rules, but unless/until ISPConfig supports configuring that, spamassassin shouldn't be installed on rspamd systems.)