Use of Macbook pro (2019) in clamshell mode

For more recent MacBook Pro models, this I much more nuanced.

Catalina software 10.15.5 and later for MacBook Pro with T2 chip (2018 models and later) includes a new feature called Battery Health Management. Based on your usage patterns, this widens the hysteresis to initiate a charge cycle at a lower level, and stop before 99 percent. 

About battery health management in Mac notebooks - Apple Support

About battery health management in Mac laptops - Apple Support

This relaxes the set points around re-charging (based on your usage patterns) and can improve long term battery lifetimes. When active, recharging may stop short of 100 percent charged. Recharging may only begin at a lower level (typically 80 percent level, variable based on usage patterns) than the previous "normal" threshold of 92 percent or less.

<< I have also been told that continually supplying power is detrimental to the battery life. >>

That is a dangerous over-simplification.

Your MacBook Pro  uses ‘smart charging’ to charge in the optimum way, and only when necessary. Plugged in is Not necessarily actively charging. There is substantial hardware and software cooperating on battery and charging issues. Simply asserting a charging voltage against one of the interface(s) will NOT successfully charge your MacBook Pro.

In general, you should ALWAYS connect AC power when it is possible to do so, and only run on batteries (which could be somewhat slower) when no AC sources are at hand. Your Mac will NEVER over-charge. 

That computer is a battery-CAPABLE device. It is not optimized as a battery-operated device. (It is NOT an iPhone.)