At least one of his points is already addressed by the twitter thread he links: Debians bug tracker behaves correctly, Gmail just ignores the information included in the email headers to group them.
So he has been told that this is a Gmail issue, but insists on using it. Meanwhile he complains that the rsync package maintainer is blocking his changes out of personal preference. Double standards much?
Gmail is an immensely popular email provider with 1.5 billion users in 2018. It has 65% of US market and even on a global level it's the leading email provider.
This is not some esoteric email client that a handful of developers refuse to let go.
But it still is the source of the faulty threading behavior. The issues also seems minor enough that there is no point in adding a hack on Debians side to make the output on googles proprietary endpoint look better.
So he has been told that this is a Gmail issue, but insists on using it. Meanwhile he complains that the rsync package maintainer is blocking his changes out of personal preference. Double standards much?