The problem with modern smartphones is that they run on battery, even when plugged into external power. You cannot use it without the batteries without some more hardware hacks. As the batteries aren't designed to be always in a fully charged state, if you decide to use the phone with the battery (with 24x7 power supply) then you also have to carefully monitor the batteries too. Batteries tend to heat up when charged / discharged, old batteries can also bulge or, worst case, even explode. Another thing you need to consider is cooling - Smartphones are designed with the idea that their CPUs won't run at 100% for a very long duration. A smartphones CPU, Camera, battery etc. tends to get hot on prolonged use. So if you are, for example, planning to use it as a mini webserver or media server or as a 24x7 streaming cam, you need ensure it remains adequately cool. And ofcourse, the best smartphones for these kind of projects is one capable of running Linux.

Some OnePlus devices have a "bypass charging" system. My OP8 Pro is running since 2y now as a Kubernetes node, and nothing bad has happened to its battery (yet).

I'm honestly curious to see if bypass charging is on, but it's been a while since I touched the device itself and it has been fine do far. YMMV

See: https://blog.denv.it/posts/pmos-k3s-cluster/

Wouldn't setting the charge limit to a really low ceiling suffice?

>>The problem with modern smartphones is that they run on battery, even when plugged into external power.

That's not universally true - a lot of android phones have a "bypass charging" mode where you run directly from the power supply, and indeed you can run them with the battery removed.