Is that wood subfloor? Maybe a buffer and a Diamabrush would do the job.

When you say 'new underlayment' what is the material you are speaking of? If you want to put down a layer of 1/4" plywood, then a simply screw-down method is just fine. If you are speaking of UNDERPAD, you may want to investigate the LVP's installation guide. Most LVP's do NOT like underpad. Some allow it, but most do not.

Let us know the name of the LVP and the underlayment you were thinking of and we will see if it is going to work.

are you sure that mastic under there does not contain asbestos?

@cat_ky The house was built in 1991 so no. Good thought though!!

What type of underlayment did you want to use?

We are using a 1/4” underlayment. Thinking a belt sander may do the trick?

And doing Provenza LVP for our flooring.

Ceno carbide shredder will work better than Daimabrush. It won't gum up or glide over thin material. It's designed more for removing mastic type material. Daimabrush will do the job also, only slower and may not remove super thin layer. My Daimabrush collects dust, since purchasing a Ceno Predator.

I'd leave it, but I've removed adhesive like that with scalding hot water.