Older node-red uses ACE editor. From version 3, Monaco editor is the default - and for good reason - it tells you about coding problems, it offers hints and built in help and it is generally much more upto date with modern JS. It has a whole bunch of handy snippets (e.g. start typing global or clone or status then hit the tab key to see how it both helps you AND gets it right every time).
If you really want to revert, search for the word monaco in your settings.js file and change it to ace, restart, refresh. NOTE: the ace editor will quite probably be removed altogether after NR v4 so (IMHO) I recommend you get used to it and improve your code.
PS: Monaco is the same editor used in the insanely popular code editor "VS Code" - it is good - I would recommend sticking it out.
