Windows Explorer Slideshow is not color managed, at least on my W10 Pro system. I created a folder with 4 images (see attached), three with profiles (ProPhoto, Adobe1998, sRGB) and one with no profile (started out with sRGB). Windows Explorer rendered the thumbnails correctly, appeared to assume sRGB on the no profile image, and Windows Photos also correctly rendered the images. Slideshow appeared to treat all images as if they were sRGB, e.g. the ProPhoto image came out close to gray.
Richard Southworth
Added by edit - I should say, Windows Explorer appears to correctly convert all images to sRGB, then displays them with no color space conversion to the monitor. On my wide gamut display the images are all shown as over saturated, as is typical on a W10 system. Same with Photos. Slideshow makes no attempt at all.