I used a 2012 MacBook Pro in clamshell mode, so did a 2018 model. Neither of them exhibited any issues. An issue I have seen repeatedly on all the others, was battery expansion when constantly being connected to power and never turned off. Some argue it’s extra heat that damages the batteries, making them expand. All of those were half the time closed while in sleep mode. When in active use, they were always with the lid open. So, dunno… 🤷‍♂️