At a Pro discussion panel at the UC with 8 of the senior product architects taking questions. They were asked when spaces would be supported in lyrx file names and it was the only question none could answer. The chief architect just said 'I'm surprised they're not already'!
Said they'd get back to me...
One of the purposes of the layer files it to make the feature class file look legible from say their geodatabase naming convention. Having to _ these does not add to the clarity i.e. 'Waste Pipe Pressure Tests' or 'Waste_Pipe_Pressure_Tests'
P.S. see my earlier point: I rarely put spaces in (prefer underscores), but sometimes, client working practices aren't always as geared to the particular idiosyncrasies of the GIS data management world. They simply want / expect an application to be able to save a named file with a space in it. Removing white space should be a matter of choice, not one enforced by the software