As it happens, yes, but it wasn't always considered a necessity -- the Skylab and Freedom mission rules were "happy" that another return vehicle could be launched to bring them back at the end of the mission. Skylab never got as far as that, and ISS didn't have the capacity for to support more than a Soyuz worth of crew while that policy was in place. The premise was found wanting following the lengthy return-to-flight following the Challenger failure.

Following Columbia, Orbiters were no longer considered to be guaranteed to be capable of returning their crew, so a stand-by STS had to be ready for launch for the duration of a STS mission to the ISS

In the event of the final shuttle mission to the ISS not being able to return its crew there would not have been enough on orbit return capacity, and the Soyuz missions would have operated on a two up three down basis at their normal cadence until the capability was restored.

As it happens, yes, but it wasn't always considered a necessity -- the Skylab and Freedom mission rules were "happy" that another return vehicle could be launched to bring them back at the end of the mission. Skylab never got as far as that, and ISS didn't have the capacity for to support more than a Soyuz worth of crew while that policy was in place. The premise was found wanting following the lengthy return-to-flight following the Challenger failure.

Following Columbia, Orbiters were no longer considered to be guaranteed to be capable of returning their crew, so the Contingency Shuttle Crew Support missions were devised, whereby a stand-by STS had to be ready for launch within 64 days of an ISS mission launch (14 days for the original mission, 10 days for a decision to launch and 40 days to prepare)

In the event of the final shuttle mission to the ISS not being able to return its crew there would not have been enough on orbit return capacity, and the Soyuz missions would have operated on a two up three down basis at their normal cadence until the capability was restored.

As it happens, yes, but it wasn't always considered a necessity -- the Skylab and Freedom mission rules were "happy" that another return vehicle could be launched to bring them back at the end of the mission. Skylab never got as far as that, and the premise was found wanting following the lengthy return-to-flight following the Challenger failure.

Following Columbia, Shuttles were no longer considered to be guaranteed to be capable of returning their crew, so a stand-by STS had to be ready for launch for the duration of a STS mission to the ISS

In the event of the final shuttle mission to the ISS not being able to return its crew there would not have been enough on orbit return capacity, and there would have been an extended series of two up three down missions to return to that capability over a couple of years.

As it happens, yes, but it wasn't always considered a necessity -- the Skylab and Freedom mission rules were "happy" that another return vehicle could be launched to bring them back at the end of the mission. Skylab never got as far as that, and ISS didn't have the capacity for to support more than a Soyuz worth of crew while that policy was in place. The premise was found wanting following the lengthy return-to-flight following the Challenger failure.

Following Columbia, Orbiters were no longer considered to be guaranteed to be capable of returning their crew, so a stand-by STS had to be ready for launch for the duration of a STS mission to the ISS

In the event of the final shuttle mission to the ISS not being able to return its crew there would not have been enough on orbit return capacity, and the Soyuz missions would have operated on a two up three down basis at their normal cadence until the capability was restored.