I'm sorry that I'm not explaining properly: I start a new message in Mail.
I attach, say, a small (60K) PDF; I send the message - to any or all recipients.
To save space in my ~/Library/Mail folder hierarchy I want to delete the attachment just sent, which - as I understand it - is actually an 'embedded' MIME part like anything else. I have always been able to do this in previous versions of macOS.
In Monterey, removing the attachment which I added to my outgoing message and which I have just sent is no longer possible because its menu item is greyed out.
I did originally set the new Protect Mail Activity on in my install. But it prevented any images in incoming emails from showing.