I believe they were called E̶x̶p̶e̶r̶t̶ ̶S̶e̶x̶c̶h̶a̶n̶g̶e̶ Experts Exchange.

Unrelated, but does anyone know of a site specializing in procedures for changing one's sex, as done by an advanced professional (an expert, even)?

The parallel is more striking than your post suggests.

Experts Exchange had no paywall for years. Endless people built the site up, provided help, then suddenly?

All their posts, content, and more were one day monetized, and locked away. Same thing happened with IMDB too, at the start. It was all, all of it, user contributed.

Then one day, they locked it all up. Of course, IMDB has grown a lot since then, and allows you to download the database if you wish regardless...

But point is, this sort of thing happens all the time...

Actually the answers were obfuscated by html/css. Dev tools could expose the answers.

Experts Exchange