Do taxi players count toward Max PF (potential points)?
Yes, they do. In the Max PF column in your standings, you'll see the amount of potential points your team could have:
This always includes any taxi players who are on your team, whether they are active or not.
They count toward your Max PF because the team owner could have activated them for that week.
There isn't an option to change this setting as it's a Sleeper default. It's a way to help prevent tanking in some leagues that take point totals into consideration each year.