I'm having a similar issue. Mine only comes on when I use the keyboard shortcut for the snipping tool (Shift+Windows+S). Sometimes the application looks like it doesn't load properly and the yellow borders appear but do not go away. I have been searching for a resolution to this but I have no seen anything as of yet.
Look for a process called Screen Snipping. It has a blue icon with an X in a box. End that task. Sometimes, I have to CTRL+SHIFT+ESC (open task manager) and then ALT+TAB into task manager, arrow up and down, and ALT+E to end it.
Not sure what happened with the last Windows update, but the WINDOWS+SHIFT+S shortcut hasn't worked since.
Narrator is off, and isn't it blue border when using narrator?
Sometimes when I press PrintScreen to launch Snip&Sketch it doesn't launch and the yellow border shows up. Pressing PrintScreen again launches Snip&Sketch as expected but the yellow border doesn't disappear after exiting it. Sometimes it disappears in 20 seconds but sometimes it stays for several minutes.
To reproduce I just mash PrintScreen and Esc, but usually it appears after launching Snip&Sketch for the first time in a while.
Not an answer but a comment. My Snip & Sketch went to 'yellow border' mode after a Windows update about a year ago. Tried all the resolutions I could find to no avail. A recent lightning strike forced me to buy new tech: a Surface directly from Microsoft with Win10 Home pre-installed. A setback but at least, I thought, Snip & Sketch will work; not so much ... still yellow border out-of-the-box (though I did apply updates first thing ... my bad?). So there is something deep or subtly wrong with this app or its deployment. Please do not remove the Snipping Tool.
My 'Use high contrast' is off which is what I assume you are suggesting; turning this on does not provide a choice of None. Trying a Snip & Stretch, alas, still gives yellow borders but no way to make a snip. Thanks for the idea!