That's commonly said on this forum, but that is not my experience.Not true.. When you adjust the charge level slider to 100%, the max estimated range is displayed, nothing more. The difference between advertised max range and this estimate is not how you understand the health of the battery, period.
For my car right now...
Sliding to the right: 289 miles
Scanmytesla (reading what the BMS thinks): 287 miles
Using estimated range for the degradation calculation: 287/326= 88% of its original capacity.
The original capacity is 77.8kWh (also reported by the BMS), so using the range estimation for battery health calculation shows my current capacity is 0.88*77.8 = 68.5 kWh. What does the BMS say? The BMS also says 68.5 kWh.
The slider range estimate is within 1% of what the BMS is reporting. This is what the BMS uses for battery capacity (a.k.a. battery health).
From what I see, you can observe battery degradation from the slider as well as the BMS can see it.