IMHO plesk is just an interface shell to manage the various open source programs apache, nginx, postfix, dovecot, bind, mysql and others.
It does so by merely manipulating their configuration files. For Plesk these programs dictate how the configurations should be.
Plesk shouldn't filter input more than these programs require.
bind fully supports underscore for use in NS records and cnames and therefore plesk should do as well.
Letsencrypt supports it... well they even require it, so who is plesk to deliberately cripple their product over semantics which do not reflect the reality of today.
It does so by merely manipulating their configuration files. For Plesk these programs dictate how the configurations should be.
Plesk shouldn't filter input more than these programs require.
bind fully supports underscore for use in NS records and cnames and therefore plesk should do as well.
Letsencrypt supports it... well they even require it, so who is plesk to deliberately cripple their product over semantics which do not reflect the reality of today.