I travelled to the U.S. and Mexico using eSIMs recently and was running into this issue a lot. I guess this was the reason, thank you.

I think you’re right about this. There should be a special “limited” icon to show when your service is being deprioritized. I think would actually help the carriers as it would tell customer when a degradation of service was due to a congested network vs low signal.

It's not really possible to benchmark real-world. But to see which priority your account is provisioned at just need ability to see lower-level information from the baseband. Deprioritization = QCI 9. Different carriers use different QCI's but 9 is lowest, 6 is usually highest priority. Network Signal Guru is an app for android that will report this information, but it usually requires root (I think the pixel phones do not, qualcomm devices usually do).

The radio layer is where deprioritization happens, not through deep packet inspection (as would be the case for video throttling). Having a strong signal with good RF conditions deprioritization won't be noticed as much, also won't be noticed if the sector isn't busy. If it is busy and you are near cell edge, that's when speeds will fall off (as the radio layer will schedule less time and resources for your device, and the worse conditions means it can't send as much data with those resources allocated).