https://www.computerworld.com/article/3690908/apples-clean-energy-charging-is-the-future-of-electronics.html#:~:text=Announced%20with%20iOS%2016%20and,it%20will%20be%20at%20night.

When Clean Energy Charging is enabled, your iPhone gets a forecast of future carbon emissions for your local energy grid and attempts to adjust the charging to times when there is cleaner energy production. That means when you plug in your iPhone, it might not fully charge if your previous charging habits indicate you won’t need to use your iPhone for a while (usually while sleeping). It’s not hurting your iPhone’s battery or purposefully keeping your iPhone’s charge below usable levels.

Even when turned on, however, Clean Energy Charging won’t automatically limit your charging. It works together with Optimized Battery Charging and Location Services to learn your usage and charging habits to ensure that you’re never left stranded without a charge when you need it. For example, it won’t turn on if you’re traveling, and as Apple explains, several settings need to be enabled for the feature to work:

https://www.macworld.com/article/1065566/ios-16-1-clean-energy-charging-battery.html