The allmail folder is where all messages are stored, duplicates of their existence in other folders. Have you tried deleting from the primary folder for the messages? That would be my approach, as using the Allmail folder is generally not recommended. It doubles your disk usage and increases cpu and resource usage for every send and receive.

I've been doing what you say for years, deleting messages from my folders, but they remain in the “all” folder. Until now, I thought that deleting messages from specific locations and the trash would remove them forever. What can I do?

I am starting to feel like a broken record on this. Does anyone read the user support documentation that says what happens, or is the concept that the folder just do not exist to hard to grasp. See https://support.mozilla.org/en-US/kb/thunderbird-and-gmail#w_understanding-gmail-labels-and-thunderbird-folders

"All mail" email can be deleted either be changing what happens when a mail is deleted to just mark it as deleted (it will still appear in the user interface with a strikethrough for a while.) This is what Google document on their website as their preferred arrangement. I just don't like it.

See https://support.google.com/mail/answer/78892?hl=en#zippy=%2Cthunderbird

Server settings tab

• Check "Check for new messages at startup."
• Check "Check for new messages every 10 minutes."
• Next to "When I delete a message," select Just mark it as deleted.
• Uncheck "Clean up ('Expunge') Inbox on Exit."
• Uncheck "Empty Trash on Exit."

Copies & Folders tab

• In the "When sending message, automatically" section, uncheck "Place a copy in."
• In the "Drafts and Templates" section for "Keep message drafts in," select Other and then Your Gmail address and then [Gmail] and then Drafts.

Junk Settings tab

• Uncheck "Enable adaptive junk mail controls for this account."

Or

Hold the shift key and press the delete key. That shortcut deletes directly and does not place a copy in the trash/deleted folder (which incidentally results in that email being restored to the all mail folder because that is where google stores ALL MAIL as they have no folders on their web site. Despite what appears to be folders in their webmail they are populated on the fly by google search using labels on the email in the all mail folder.