Can series 1 still pair with iPhones

Theoretically yes. With luck and a tail wind.

If you have iOS16 or later on the iphone, series 1 to 3 Watches are difficult to pair because of relatively low storage but…

Ensure the iphone is on iOS16.4 or later.

Erase the Watch completely. Put your watch on its charger and keep it there until you complete these steps. (select the watch OS version) Erase Apple Watch – Apple Support (UK) 

When pairing do not try to restore the backup, pair as a New Watch Set up your Apple Watch – Apple Support (UK)

Before iOS16.3 I would have said NO you can't pair a series 1 to 3. Memory too small, Buy a new one.

That was the standard response early on in iOS16.

This way since16.3.1 it should pair my way and has been shown to work.

If you have already unpaired then you have a backup. Try setting it up normally and restoring the backup. If it fails you have my method to try

Good luck.