There's a lot of history behind WhatWG that revolves around XML.

WhatWG is focused on maintaining specs that browsers intend to implement and maintain. When Chrome, Firefox, and Safari agree to remove XSLT that effectively decides for WhatWG's removal of the spec.

I wouldn't put too much weight behind who originally proposed the removal. It's a pretty small world when it comes to web specifications, the discussions likely started between vendors before one decided to propose it.

Probably if Mozilla didn't push for it initially XSLT would stay around for another decade or longer.

Their board syphons the little money that is left out of their "foundation + corporation" combo, and they keep cutting people from Firefox dev team every year. Of course they don't want to maintain pieces of web standards if it means extra million for their board members.

I think the person you’re replying to was referring to the partial support of XML instead of the xslt part.

Then standards body is Google and a bunch of companies consuming Google engine code.