You can walk under the Arch of Titus, I did
The Arch of Arch of Septimius Severus I was not able to get that close
You cant walk under the Arch of Constantine either , Its fenced around.
The arch of titus is in the Roman Forum , The whole area is a ruin anyway, Its an archelogical dig area. It sort of feels like almost being in an area that was bombed.
There are Fascades to Roman Temples there like the Temple of Saturn, but its only the Fascade its like a homage to a lost civilization.