The iOS community has been wondering if pairing the iPhone 6S with the Series 1 Apple Watch is worth it even if it would cause them to lose the jailbreak.
If you have your iPhone 6S jailbroken, the iOS version should be at 10.2. However, you should at least have your device updated to the latest version before you can pair it with the Series 1 Apple Watch.
Thus, before pairing your iPhone 6S to this type of watch from Apple, you have to consider the following conditions.
In a recent update released by Yalu jailbreak team, iOS versions 10.3.2, 10.3.1, to 10.2.1 may not have a chance to be jailbroken yet. Moreover, although the Pangu team released a demo for an iOS 10.3.1 jailbreak, it is not a guarantee that it would be released to the public anytime soon.
Therefore, if you really want to pair your iPhone 6S to your Series 1 Apple Watch, you need to sacrifice losing your jailbreak.