Não é mencionado nas notas de lançamento, mas o auth_imap foi removido do núcleo para um plug-in instalável separadamente. Consulte o link :
Possible issues that may affect you in Moodle 3.4
- The following authentication plugins were removed from core and can be found at the following URLs. If you use them, you must provide them manually during the upgrade (MDL-50925).
- FirstClass (auth_fc) - https://github.com/moodlehq/moodle-auth_fc
- IMAP Mail server (auth_imap) - https://github.com/moodlehq/moodle-auth_imap
- Network News Transfer Protocol (auth_nntp) - https://github.com/moodlehq/moodle-auth_nntp
- Pluggable Authentication Modules (auth_pam) - https://github.com/moodlehq/moodle-auth_pam
- POP3 Mail server (auth_pop3) - https://github.com/moodlehq/moodle-auth_pop3.
(Pessoalmente, eu sugeriria um sistema de conexão única SAML2 ou LDAP.)