p5.js implements the Processing API in JavaScript – you program it using JavaScript syntax.
Processing.js – http://processingjs.org/ – is a transpiler. It runs in JavaScript, but you program it in the Processing (Java mode) syntax, which it converts into JS behind the scenes. (Keep in mind that originally, Processing (Java) itself ran in the browser using the fairly ubiquitous Java applet plugins. Then, gradually, it didn’t.) So for people familiar with the Processing Java desktop mode, they can take their code, paste it into a Processing.js, and under the hood it magically becomes JavaScript and runs (90%, with caveats and tweaks, of course).
The last Processing.js release was 1.6.6 ~2017, but the website was last updated in 2014, which contributes to the impression that it is old. Even more, the key thing is that the mode plugin doesn’t ship with Processing 3, and the version in contributions manager is only compatible with Processing 2. That makes it feel “neglected” for those that want to write Java-mode-for-the-web on the desktop – you have to install Processing 2 to use it.