Very useful and up to date PDF available from the ROMA Pass website - if you have a pass (or are planning to get one) you should refer to this to see what the current situation at each location is due to COVID-19.
It has details on mask guidelines, reservation suggestions (even with the pass), etc. Last updated on the 15th of September. For the Ara Pacis Museum the current suggestion is "Reservations highly recommended also with Roma Pass".