Why is the "Battery Health..." button missing in my Big Sur System Preferences?

The Battery Health... button requires hardware support as well as being provided in macOS software. Older MacBooks do not have the hardware capability.

For example, my MacBook 2016 with macOS 12 does not show Battery Health and does not support Optimised Charging.

But you can find out about the battery health. The app coconutBattery does this. As well as showing current battery health it will keep a history for you.

I am taking 'battery health' as the maximum charge capacity of battery now compared with nominal value when new. Typically this will decline from 100% (or even a bit more) when new to around 80% when you should be considering a battery replacement.

Edit (added, with thanks, from @AVelj's comment):

For alternative ways of getting battery health/condition, you can access this information from System Profiler>Hardware>Power and it should tell you what is the maximum charge of your MacBook. You could also run the command system_profiler SPPowerDataType in terminal to obtain this information.