Apple Watch 5 compatibility with S model phones

Yes. Watch - Apple states:

• Apple Watch Series 5 and Apple Watch Series 3 require an iPhone 6s or later with iOS 13 or later.

AppleWatchCT wrote:

Thanks. I was concerned when I heard rumors that certain features like the ekg didn’t work as well with the S models.

You are welcome. The primary consideration with ECG capability is that it is not available in all countries.

1. watchOS - Feature Availability - Apple
2. Taking an ECG with the ECG app on Apple Watch Series 4 or later - Apple Support

Yes it will.