Why, it's basically charging it unnecessarily, which is how I messed up my iPhone 6's battery.
On the way to work for 30 min, and another 30 min back towards home, I don't really use it much at work, so it will never drop below 80% before charging again.
My iPhone 6 is so fucked up, it takes a long time to drop from 100% to 90%, then drops abruptly until 60%, then slow until 40%, and after that it tanks straight to 15% lol.
Nah, not really. When it is "plugged in" it isn't using battery power it is using the AC from the wall and also is smart enough not to feed the current to the battery when at 100%. It's a smart phone, it's smart enough to control the juice getting into it, most modern tech is.
As for your 6,
ALL phone batteries have finite life, they are not designed to last forever. Any lithium-ion battery does lose significant storage over time and starts to act up. Generally, you can expect get 400 "charge cycles" in your phone before the battery starts acting up. You complete one charge cycle when you've used (discharged) an amount that equals 100% of your battery's capacity — but not necessarily all from one charge. For instance, you might use 75% of your battery's capacity one day, then recharge it fully overnight. If you use 25% the next day, you will have discharged a total of 100%, and the two days will add up to one charge cycle. It could take several days to complete a cycle.
In other words, your 6 battery has likely gone though so many cycles that is is no longer optimal. If you use your phone every day, you can generally expect 1.5 years of full lithium ion service before the battery starts to degrade. The 6 came out in 2014 which means if you got it at launch or even the following year, you're reaching the charge cycle max - You can replace the battery or get a new phone.