There’s a TechCrunch article that says:
Out of the box the phone comes with Android 9 preloaded. A post-launch update will make it easy for buyers to wipe Google services off their slate and install the Android Open Source Project instead.
and
The handset comes preloaded with a vanilla implementation of Android 9 (Pie). But as noted above buyers will be able to switch for a non-Google alternative — via an updater that will let them wipe and install the Android Open Source Project flavor of the OS. (The updater will come post-launch, according to van Abel, who notes that around 5% of Fairphone users opt to go full open source.)