I recommend moving towards Pulp 3 since that is where all the development effort is going and Katello 4.0 drops support for Pulp 2. Katello 3.18.* is the last Katello version with Pulp 2 support.
“supported, but overridden to use Pulp 2” is a configuration file setting.
If you have a look at about minute 53 of this video you can see me changing the setting. If you want to do a Pulp 2to3 content migration before you switch, you may want to start at around minute 45 of the video. 
You can also read the “Pulp3 Migration” instructions here: Foreman :: Plugin Manuals