Is you account IMAP? I see folk here all the time asking question exactly as you are. I assume it is because they are either not aware of, or do not grasp the significance of, the Synchronized part of the IMAP mail account.

Regardless of the settings in Thunderbird, the IMAP server is king of the mail pile and if your provider deletes mail after XX days or empties the trash every night. Thunderbird will update your data to match the synchronized status of your mail. If you phone only has a very small memory for messages and deletes them after a couple of days, then that will synchronize also to Thunderbird via the mail server iof the phone is using IMAP.

Is there a way to see which emails have been deleted after the retention policy (RT) has been applied?

I set RT to keep in "all mail" of my gmail account ONLY the 8000 most recent emails.

First of all, the RT seems not working. Right now I have more than 8400 emails this means that it is more than 5 days that the RT does not clean up this folder.

Second, if I force to "compact" the folder, I can see that the emails decrease to 8000, but it seems that the oldest ones are not deleted. Is there a way to know which emails have been deleted?

Thanks for your support, Massimo