I found this comment from over 6 years ago. It has a hyperlink to a YouTube video, not a link, but a hyperlink. How was/is this done?
That link is directed to a video and highlights a comment...
This link https://www.youtube.com/watch?v=bbDHfUc_fMI&lc=UggS2CQUfc2M4XgCoAEC is normal until the &lc=UggS2CQUfc2M4XgCoAEC
the &lc= is basically what dictates that a comment will be highlighted
People can get to a link like that by notifications that highlight there own comment. What probably happened was the person who shared that link, shared it when they were looking at there own comment.