Both the smartwatch and the smartphone must have the latest available version of iOS at the moment of joining. As at September 2022, these software versions are iOS 16 for the iPhone (not supported on iPhone 7 and older iPhones) and watchOS 9 for the Apple Watch (not supported on Watch 4 and older).
If you have an iPhone 6s or iPhone 7 and have already used the MultiSIM service with it, you can continue using the MultiSIM service going forward. With these phones, you are also able to subscribe to the MultiSIM service if the Apple Watch software is older than watchOS 9, but in this case, the Apple Watch will not work when roaming.
If you update the software of an Apple Watch paired with an iPhone 6s or iPhone 7 to watchOS 9 or later, you will no longer be able to subscribe to the MultiSIM service. In this case, you need a newer phone (iPhone 8 or later). You can see which versions of iPhone and Apple Watch are compatible with each other on the Apple website.