This site seems like a decently credible source on battery life, and they recommend setting devices into a mode to leave the charge at around 80% if its not going to be really used heavily as a mobile device.

https://batteryuniversity.com/index.php/learn/article/how_to...

"A laptop battery could be prolonged by lowering the charge voltage when connected to the AC grid. To make this feature user-friendly, a device should feature a “Long Life” mode that keeps the battery at 4.05V/cell and offers a SoC of about 80 percent."

I've had a lot of laptops with this kind of mode on them from several different manufacturers. I think Samsung, Lenovo, and HP have some knowledge of extending the life of their batteries. https://support-us.samsung.com/cyber/popup/iframe/pop_troubl...

"...it is strongly recommended to select "Optimize for Battery Lifesapn mode" or Conservation Mode and keep AC adapter connected alll the time. This mode will enable the battery to be fully charged to 80% or 60% of its design capacity." https://support.lenovo.com/us/en/solutions/ht069687

I have little reason to think a similar concept applies to phone batteries, as they're usually a similar chemistry these days.