The helper is correct in that add-ons do need to be updated to work
with new versions of Thunderbird ( ditto for Firefox ) and it's the
original writer who has to do it. Not all add-ons make the change,
whether due to the writer no longer being around or unable to alter
the code to suit.

Regarding 'send with blank subject' I wonder why you want to do that
anyway ? Blank subject lines are often rejected by mail servers as
it's indicative of spam / virus sent e-mail. Is is so onerous to type
in a subject line ??

The GMinder add-on you list is an MSI file which is a Windows one, not
for Linux. Did a but of research and there's a way to get it working
for Evolution but not Thunderbird. Not sure if it will work in Wine
but worth a shot ?