Disclaimer: I'm not 100% sure about this, but the following would be my understanding.

synchronize the most recent 30 days

This should only be relevant for initial sync. E.g. to avoid synchronization of thousands of messages from an email archive dating back for years. After initial sync of the most recent 30 days Thunderbird should continue to sync new messages just normal. No messages should be deleted.

Deleting of messages is controlled via retention settings.

thanks for your answer christ1. As it would be only for initial sync that answers my question but I didn't really mean deletion of the old messages on the server but removing the cache of the downloaded synced header.

I didn't really mean deletion of the old messages on the server but removing the cache of the downloaded synced header.

In the Thunderbird context the term IMAP 'synchronization' refers to downloading an offline copy of messages to the local disk, so that those messages can be accessed without a network connection to the server.

It is not meant as synchronizing the messages between the IMAP server and Thunderbird (i.e. downloading message headers). That is what the IMAP protocol does per-se.

So no, I'd not expect Thunderbird to delete anything after 30 days due to the "synchronize the most recent 30 days" setting, neither cached headers nor offline copies of previously synchronized messages.