Apparently a notification will show up when optimised charging is enabled. I’ve never seen it, because the functionality also requires Significant Locations and other location services (listed near the bottom of the page) to be enabled.

That seems to me to be quite unfriendly to privacy. And given that I always charge my phone overnight and use my phone’s alarm function, it could simply use that as the target time to finish charging to 100%. No complicated location-based ML needed.