I'd imagine there's a lot of undesirable friction in "tipping" online (clicking a link, filling out a credit card form, etc.) that a tip jar in a cash-based society never had.
> I'm wondering what is most commonly used for tipping websites/content?
Probably something like ko-fi.com. I have it on my sites, but no one has ever tipped (besides my girlfriend, haha). I imagine other people do get tips via kofi, though.