Do iPhone battery extenders reduce regular battery life?
In theory, no. Battery extenders work just like a charger. The main problem with these extenders, assuming you are not buying some top notch apple certified extender, is the different random voltage they output and that can seriously damage your battery and device.

If you are getting a battery extender I would recommend getting something that is apple certified or at least branded from a known company. Stay away from all the knockoffs AliExpress garbage.