Huge thanks to Marijn for all the work he's put into this.
We're currently integrating CodeMirror 6 into Overleaf [1], and you can try it out by joining our beta program [2] (which will give the option to select the beta source editor, which is the one built using CM6). If you do give it a try and spot any bugs/issues, please let us know, feedback is always much appreciated.