Occasionally we do user management in Pardot. Only recently did we notice that for several users we see the message "Please select a CRM Username for this user" on the top of the window.
We have no more Salesforce licences left and, to our knowledge, we never needed a Salesforce license for each of our Pardot users.
Googling led us to the "Salesforce User Sync Enablement Guide". It turns out that somewhere in 2018(?) a feature was added called "Salesforce User Sync". Disabled by default and indeed, I checked it, disabled in our org.
So who knows what has happened to the user accounts that existed only in Pardot? It seems they are no longer working. The "Salesforce User Sync" feature promises Synced Users and Pardot-Only Users, so that sounds good, but no feature comes without catches, so what is the catch here? And is our assumption right that, without the feature enabled, Pardot-Only users are not usable anymore?