The developers plan to implement such a feature. https://discuss.grapheneos.org/d/9017-how-can-i-limit-battery-charge-to-80/34
I am very happy this feature is being added as I will no longer have to use root just for it. I would also like to share my ideas on how this should be implemented:
It would be an opt-in feature. Opting in will enable everything:
First there would be a basic configurable battery charge limit. The default settings will stop charging at 80% and restart charging if it drops below 78%.
Second there would be a configurable max wattage the phone can charge at, with a default value of 10w.
Third there would be a configurable wattage limit at certain temperatures. The default settings will start reducing wattage from max wattage (defined in setting above) at 30c to no wattage at 40c linearly. So with the default settings, the phone will only be charging at 5w when it reaches 35c.
I believe implementing it this way will keep it simple, while offering all the customization anyone could want, while also having very effective defaults for battery longevity and convenience.