How to remove iPhone XR motherboard bottom shield?

These shields are not the easiest to remove as they hold a fair bit of ground plain. I use full heat and air flow with a large or no nozzle. The shield you are talking about should come off when the hot air temp reaches around 400 degrees.

All hot air stations are different; the temperature displayed may not be accurate due to the age of the machine or its make.

You are right thinking that exposed heat might damage the motherboard. The other method you can use is to cut it off. I see a few that come in with prior repair attempts which have had the shield removed this way.