I think, the changed if-clause condition now leads to new message header ‘list-post’.
Example new mail from our discourse shown in thunderbird:
and mailheader has line:
List-Post: mailto:replies+6e23071765f50da1c819aba7f2ffb386@forum.sol....
Before new release (no Reply List is shown):
and mailheader has no line with “List-Post”.