Older items in the Trash folder used to be automatically deleted after a few days. About a couple of weeks ago, this stopped happening and I don't know why. Does this affect anybody else? Has some process on the BT Email servers stopped running?
I use IMAP with clients on several devices. The only change I am aware of around this time is installation of iOS 13 on my iPhone. This seems to have re-named the Trash folder to Bin, although it clearly still maps to Trash on the BT Email servers.