A number of journals actually have clauses around how you can’t publish it anywhere else if they accept it.
So you can’t ‘publish’ it in those places, but you can send it privately to people who ask.
It seems like that could just about go in one’s email signature:
“If this message has an attached published paper, please do me the service of making this publicly available via arxiv /scihub or other agency as I’m typically bound from doing this by the publishers conditions”
At least where I live the laws are such that publishers can claim copyrights only after they added their “editor” customizations such as publisher logos, page numbers, layout changes etc.
The manuscript that you/the scientist wrote and handed in to the publisher is free of that, the publisher cannot claim any rights at that state. So you always have the right to publish the “unedited” manuscript anywhere including researchgate, arxiv, your website etc.
