At Ratings of coax antenna switches I showed characteristics of a home made switch which has very low InsertionVSWR, but poor isolation.
A couple of correspondents have offered an explanation that the unused port must be shorted to get good isolation.
If that was the case, then we would expect all coax switches that leave the unused port open to have poor isolation.
Let us look at a very good coaxial relay
Above is a Dowkey 402 series relay which has good performance to GHz. It does not short the unused port.
Above is its InsertionVSWR to a mere 500MHz, it is very low.
Above is S21 to the unconnected port. Isolation is very good.
Like most coax relays, this relay leaves the unused port open, and yet it delivers stunning isolation, better than most coax relays.
Shorting or grounding the unused port(s) of a coax switch is not essential to RF port isolation, though it may have other benefits and disadvantages.