so only time will tell.
You have it backwards. When the USB charger voltage droops, the phone (or other recharging device) will dissipate more heat as the phone's charging process becomes less efficient. Apple's USB chargers generally don't sag like some cheap equivalents do.
It's easy to monitor this using simple and cheap USB tools like this one which report both voltage and current:
https://www.amazon.com/PortaPo...
(or many other equivalents)
Charge control is done within the phone, generally by a dedicated chip which also monitors battery temperature. In some cheap 'power bricks', the charge control is done via a microprocessor rather than a dedicated charge control chip, but this isn't common in more valuable products.
Li ion batteries have a separate protection circuit which monitors over voltage, under voltage and over-current (either direction). It opens the circuit when triggered.
Battery safety is controlled by IEC 62133, which was generated in the EU, but has been adopted in the US, too.
You can tune a piano, but you can't tuna fish. You can tune a filesystem, but you can't tuna fish. -- from the tunefs(8) man page