Should I plug in while playing games on iPhone 7 Plus?

In some ways it does not matter, other than the number of cycles it adds to your battery. Battery life is ultimately a function of how rapidly you accumulate full discharge/charge cycles. So if it is convenient to plug it in while playing a game or doing anything else, by all means go ahead and plug it in. That way, the phone will still be charged when you do go to leave and take it with you.

Your iPhone or iPad, when plugged in, will start charging the battery. Once the battery reaches full charge capacity, charging stops, and then yes, the device is running purely on the external power supply. So there is no active discharge of the battery.

Philly's point is simply that normal use will not kill your battery prematurely - it will just kill it normally! In other words, if used normally an iPhone battery is rated to have an effective useful life of 500 discharge/charge cycles (or maybe it is up to 1000 now, I'm not sure about that). A (normal) cycle is a cycle - so however long it takes to get there is the lifespan of the battery.

The main negative thing that will kill a battery rapidly is frequent deep discharges - all the way down to the point the device auto powers off. Avoid those as much as possible, by simply charging the device whenever its convenient to do so. Also avoid extreme heat (e.g. leaving it baking on the seat or dash of a car in bright sunlight).

It's actually somewhat simple. Usage = loss of longevity. If you have reason to use the battery, you will Impact longevity. If you want the battery to last an exceptionally long time, don't use it. ****

Also worth noting that the Lithium chemistry on which these batteries are based is inherently unstable. So "if you don't want to wear it out, don't use it" won't work: after awhile (couple of years in the phones' batteries) the things will croak and need replacement anyway. So use it and enjoy it while it lasts.

Now, how many cycles does your battery have? Maybe some battery minding iOS app states it out, the only way I've found thus far is via the Coconut Battery utility that runs on MacOS in your Mac computer. If it is running and you plug the iGadget in, an extra window pops up with all the intimates of the device.

True, its been several years since Apple removed that recommendation (may have been removed about the time of the iPhone 5 or 5s ?). The “smarts” in their SMART lithium batteries are self calibrating now, so all any deep discharges accomplish nowadays is to hasten the degradation of the battery.

Doesn't matter. Michael Black's response is correct.

I remember at one time that Apple recommended complete battery exhaustion as a means to calibrate the battery level meter. That hasn't been true for quite some time.