Where is the PlugIn for the Game Center?

Looks like not. I asked author of the Construct Master Collection about his plans to develop it and he said that he is already working on it but unfortunately you have to pay another 99 or 199 dollars to get it.

I don't know how this plugin was abandoned cause ios it is the main mobile platform in the world and every cross platform engine first of all have to support main functions of the all main platform especially if the engine not free even if not a lot of people use it. But ok, waiting realisation from the 3rd party developers or you can use cordova plugin for this but in this case you would need to create your own facade for this functionality on vanilla JS

just adding that this was a bummer to lose :(

Many games on Apple Arcade support Game Center features like leaderboards and Achievements and it actually provides a level of polish and interactivity that is desirable / helpful for exposing your game to players' friends (non-players can see what achievements / scores their friends are getting in your game, which could entice them to install it themselves to compete).