How to use an user profile custom field using Rules?
... since the Field select list has disappeared and custom fields are hidden in the Data selector suggestions.


The above is a partial quote of your question. IMO that's not correct. Instead it should say "... since the Field select list does not appear yet and custom fields are not shown yet in the Data selector suggestions".

And similar to the (my) answer to the question you linked to: those or some of the things that are still missing in the most recent D8 version of Rules. Refer to #d8rules status update November 2017 to get a better understanding of the current status of Rules for D8.

One never realizes what has been done, only what remains to be done ... So start here if you wonder how to contribute.