I think I might know of a way to simplify this. If you made the format of the message be:

TAG_NAME = "issue-number
CLOSING_TAG = f"<!-- /{TAG_NAME} -->"
BPO_MSG = f"""{{body}}
<!-- {TAG_NAME}: bpo-{{issue_number}} -->
{{message}}
{CLOSING_TAG}
"""

Then you can do:

if CLOSING_TAG not in body:
    ...

That way the search is a straight-forward string search and you leave the re module out of it. And by embedding the issue number in the comment you add a little bit of extra detail that may be useful later (e.g. if someone adds support to detect a change of issue number in the title and wants to update the comment accordingly but doesn't want to worry about any format change in the message itself).