I have not seen a chart, per se. But, PCl does give a lot of information on their excursions and on the details. Look at the details of how long on the bus, the time at the site, etc. Pompeii, probably strenuous, due to the walking. It is relatively flat, but some uneven ground areas too.
The amount of time at the site, and walking are probably why. I’ve taken some that said they were strenuous, but they weren’t in my mind.
I’ve booked and then once OB, checked with Shore excursions about my questions. They were very good about making changes, if available.