It can do damage, but is less likely to damage a loud muffler than it is a nice quiet stock type.
If they're the straight through glass pack type, they'll likely never be hurt by it other than maybe needing recking sooner. baffle types, especially restrictive stock types are slower to release pressure so it builds internally faster than they can release it and damage can occure quicker.
Once had a willys jeep, old 1946 flathead cj-2a.
When I bought it I could get it to fire up but it would only rev to maybe 1500-2000 rpm, played with it a while, checked plugs cap/rotor exhaust etc. Finally sitting there at my friends house he said just hold it down and I'll play with timeing etc. so I fired it up, put the pedal on the floor, two seconds later heard a BOOM! and a smash of glass. Looked back and saw that the back plate of the muffler and a foot or two of tail pipe had gone through his window and into the TV dropping big chunks of mouse nests and a couple dead mice along the way.
Ken.