The rule "DHCP Client", that allows
546/tcp(ip6tables) appeared in the Plesk Firewall after Plesk 18.0.38, why?
It was implemented in order to resolve the bug PPPM-13164:
It is now possible to get an IPv6 address via DHCP on Plesk servers with Plesk Firewall enabled. (PPPM-13164)
Plesk servers deployed on AWS no longer lose IPv6 connectivity once Plesk Firewall if enabled. (PPPM-13164)
This rule can be safely disabled or altered at your discretion in case your server doesn't use IPv6 via DHCP.