Gmail recommends 'Just mark it as deleted' for the delete action:

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

This is because moving a message to Trash will move all copies of the message in any folder where it's copied.

Deleted messages are shown in All Mail if that folder is subscribed (which it usually is not).

http://kb.mozillazine.org/Using_Gmail_with_Thunderbird_and_Mozilla_Suite (Labels & All Mail section)

Thanks sfhowes.

I configured the Thunderbird account: Server Settings > When I delete a message > Just mark it as deleted.

In Thunderbird, when I select an email and hit the delete key, the message is marked as deleted. When I click away from the gmail account to, say, view the folder of another email account, and then come back to the gmail account, the messages that were marked as deleted are gone.

Are the other Thunderbird configuration settings I need to make so the deleted messages are retained?

If you don't want to remove messages when they are deleted, it's better to move them to another folder. Otherwise, if you set the option to mark them as deleted, they can be found in the All Mail folder. Or, add them to Trash by drag and drop. Marking as deleted will still function as you've found when the Delete key is pressed.

How does one access the All Mail folder? I do not see it listed in the panel on the left of the Thunderbird screen where other folders are shown.

Thanks.

Right-click the account in the Folder Pane, Subscribe, Refresh, and check the box for All Mail (see picture). If it doesn't appear in the list, enable it in gmail webmail settings.

Thank you. The All Mail folder is not in the list when I right-click on the gmail folder in Thunderbird. I looked in the gmail web Settings > Labels and find All Mail "Show in IMAP" was already checked.

What else needs to be configured?

I can't say why it doesn't appear in the list. Try opening the profile folder from Help/More Troubleshooting, Profile Folder, close TB, delete or rename folderTree.json, then restart TB and Subscribe, Refresh, and see if it appears.