I’m afraid the answer remains the same. At present there is no way to force a link in a markdown file on GitHub to open in a new tab/window. The person clicking will have to decide that for themselves.
Thank you canuckjacq for your feedback.
I believe this would be a good improvement for the future. This type of UX is very bad for the website itself, since the user suddenly leaves it (I got this opinion not by myself, but by senior web designers)
In the meantime I will circumvent the pb by simply offering the user to copy/paste the URL as a string in their browser.
Cheers