I also run a few popular open-source projects (eg. Nodemailer) and what grinds my gears the most are the support requests disguised as bug tickets. It takes so much time to handle these – first to verify that this in fact is not a bug, and then come up with some kind of response.

For example - someone uses Nodemailer and the server they are running their code on has the firewall configured to block non-HTTP/S ports, including email ports. Now their app gets timeouts left and right and the obvious thing to do in that case seems to be to go and file the 1000th bug ticket in Github with the same "works on my developer machine, but not in the server" subject.

Very much the same here. I'm the maintainer of Outline (https://github.com/outline/outline) – which uses Nodemailer by the way, thanks! – and folks still push through all of the GitHub workflows that try and direct self hosting support tickets to the discussions board and post them as bugs instead . This is much more annoying and time consuming than the +1 comments which can largely be left without a direct response.