Is 'Add your 2 cents' a reference to anything because I see it as the placeholder for comments and don't understand what it means?

✋ I thought it was subtle way of asking us to donate for devRant 😅

It is an archetype of "penny for your thoughts", the nature of which is derived from the 16th century idiom, "penny for your thoughts" - the reference being an uninvited opinion.

The other possible reference is that the "penny for your thoughts" actually derives from the tuppence statement, with the latter being derived from the biblical story of a poor woman giving her last two coins to charitable causes in the face of rich families donating many thousands. This premise implies humility, the frame of which implies a small or tentative statement.

It was something my Spanish teacher used to say last year. After she gave us her opinion she would say, "And that's just my 2 cents."

I thought it was a Millennial thing.