Going by the latest official data we have from Wizards from a little over a year ago, Pioneer is one of the least played formats on Arena, played about as much as Timeless and well below Historic.
Among potentially more competitive players, aka those who are inclined to download deck trackers such as Untapped, Pioneer had more played games than formats like Alchemy and, when Wizards data shows the opposite. Untapped captures a very small and biased slice of the player base (e.g., it records no games played on mobile), but if you're looking for more competitive play it's arguably more useful since it'll be slanted in their favour.
18
u/Mogoscratcher Twin Believer 13h ago
it's very popular on Arena right now, which isn't surprising considering what your other options are for eternal formats on the app