Can commander tax be proliferated?
No.

Proliferate only deals with counters.


  701.26a To proliferate means to choose any number of permanents and/or players that have a counter, then give each one additional counter of each kind that permanent or player already has.


The commander tax has nothing to do with a counter. A counter is a physical object that is used when an effect tells you to use one.


  121.1. A counter is a marker placed on an object or player that modifies its characteristics and/or interacts with a rule, ability, or effect. Counters are not objects and have no characteristics. Notably, a counter is not a token, and a token is not a counter. Counters with the same name or description are interchangeable.


The commander tax does not use counters at all.


  903.8. A player may cast a commander they own from the command zone. A commander cast from the command zone costs an additional {2} for each previous time the player casting it has cast it from the command zone that game. This additional cost is informally known as the "commander tax."