Hello.
Yes. I would recommend that you do any Apple Watch update no matter what, because in the process, they could be fixing some important bugs. Even though that seizure feature may not apply to you, you will still want to update. If you don't update to this software, you really cannot update to the one after that (which is probably watchOS 10). My advice is to always be up to date on any software, because any update can be important in some way.
Also, it does not really matter what exact iPhone model is connected to update the software, so this feature about seizure detection would not be affected by your iPhone model.
I would like to make one suggestion, since it looks like you are upgrading your older models of Apple devices, you may want to consider upgrading your Apple Watch soon, since the Series 4 may not be eligible soon for the latest software. It is just on the cusp of being pushed off from the software. Just a suggestion, from my point of view.
Hope this helps.