I can confirm iOS 18.2.1 has no included IMAP IDLE fix. To verify this assertion I did:

1. quit iOS Mail app AND shutdown my iPhone.
2. updated and commented out the imap_capability flag on my dovecot server: conf.d/20-imap.conf (commenting it out means the defaults (including IDLE) will be supported).

Override the IMAP CAPABILITY response. If the value begins with '+',

# add the given capabilities on top of the defaults (e.g. +XFOO XBAR). #imap_capability =IMAP4rev1 LITERAL+ SASL-IR LOGIN-REFERRALS ID ENABLE STARTTLS XAPPLEPUSHSERVICE ...

3. reloaded dovecot sudo doveadm reload

4. powered up iPhone and launch Mail app. I observed NO emails would appear.....NONE....

To verify this is the culprit, I reversed the steps above and immediately could see my Inbox populated.