I just checked online, found some links for Suicide Chess, but no federation or other governing body.

So let's look at some details.

First, we are focusing on the "popularity" of variants, a bit nebulous, but I think we both can agree on some common ground.

I guess my first question is, how many people play Suicide Chess? Is there a way to estimate this? I am not sure "tens of millions" is a viable answer for this variant.

Surely if there were "tens of millions", even if 1% of these players banded together to duke it out regularly we might expect to see a federation formed with some multiple of 100,000 players.

Or can something be very popular and yet have no centralized governing body? I don't think there is a World Jogging Federation, although groups such as the New York Road Runners' Club exist.

I am not sure there is a universal answer. I can quantify sales. I can quanitify interest in the form of email contact and other communications. I can attest to people feeling very strongly about the game of Gothic Chess.

I would think such a combination would allow me to say that this is a very popular variant of chess.