Does it do this with 52.1.1 started in safe mode? https://support.mozilla.org/en-US/kb/safe-mode-thunderbird

Yes it does not delete it using 52.1.1 No need to start in safe mode as this is an instance set up specifically to delete mails from the server of two domains and it has no add-ons, plug-ins or extensions.

Wayne Mery said

Does it do this with 52.1.1 started in safe mode? https://support.mozilla.org/en-US/kb/safe-mode-thunderbird

Yes it does not delete it using 52.1.1 No need to start in safe mode as this is an instance set up specifically to delete mails from the server of two domains and it has no add-ons, plug-ins or extensions.

Did it ever delete the e-mails after 180 days, or is this a new development?

If it never did work, then I would say, yes, that is a limitation. POP was never designed to not delete e-mails from the server for such a long time.

Perhaps you should look at a server-level way to do this. Other people here report using such a method for old messages. A recent person said that they have their server automatically delete all messages over 7 days old. (I'd hate to be one of their customers).

Wayne Mery said

Does it do this with 52.1.1 started in safe mode? https://support.mozilla.org/en-US/kb/safe-mode-thunderbird

Not deleting in safe mode or not. Also not deleting after 14 days from another server with a different domain. This feature may not be working at all.

Bruce A. Johnson said

Did it ever delete the e-mails after 180 days, or is this a new development? If it never did work, then I would say, yes, that is a limitation. POP was never designed to not delete e-mails from the server for such a long time. Perhaps you should look at a server-level way to do this. Other people here report using such a method for old messages. A recent person said that they have their server automatically delete all messages over 7 days old. (I'd hate to be one of their customers).

It never did not work when set for 14 days on one server and 180 on another although the webhost on the 180 days setting claims that it is working.

It never did not work

That is a double negative. People erroneously believe that a double negative is an emphatic negative, when it actually means the opposite.

I'm going to assume that you meant that it never worked in the first place in both instances.

although the webhost on the 180 days setting claims that it is working.

That statement is vague. The host says that the setting is working?

Perhaps there is a language barrier here?

Just to make sure, are you using the setting as in the screenshot below?

Maybe you can set up something in your webmail to delete these old messages instead.