Hi all. Newbie here. I appreciate any help you can offer. In a house built in 1977, we took out a bunch of walls and will be putting in engineered wood floors throughout. I was told that I should pull out this particleboard underlayment. The floors seem solid. Don’t make noise. They are level except for the transitions between what used to be the different rooms. The biggest difference would be the transition circled in red of which one floor is 1/4" higher than the other. Part of the difference is because as the higher floor approaches the former transition, it slopes up slightly as it nears it. Overall the other room is slightly less than 1/4" higher. Sorry, I can’t see how to insert an image to show the image with red I referenced.

Although particle board is denser than most solid wood sub-floors it is only an underlayment. When particle board gets wet all the holding power for fasteners will be lost. It can swell and never shrink down to it's original size.
For these reasons it is recommended that hardwood floors never be installed on top of it by the National Hardwood Floor Assoc.
If a floor is fastened over it you may never have a problem, but you never know what may happen.
The hardwood floor is a valuable investment. Is the risk worth leaving it in place and installing a floor over it when it may only take a day or so to remove it along with all the fasteners?
If you need to get the floor flat you can use thick tarpaper in layers no more than a quarter inch thick or plywood that could be sanded down where it blends with a higher sub-floor. Check with a ten foot straight-edge. You ought to be able to get within an eighth of an inch to flatness.