but notice, it's not just a single toggle button, in fact it's an item of a longer list, or group. also obvious when you look at that part in dev tools.
another sign is that at the top it says "Context Menus"
yes, plural, more than one.
my guess is that more of these bubble/popup style menus will show up soon. the next place is probably going to be Immersive Reader mode, where you don't have to right-click on highlighted text anymore, or keep going up and down to use the toolbar items repeatedly.
you just highlight a text and all the available options will automatically be presented to you.
it could also hint at customization abilities that might be coming to the regular right-click menu and maybe let us customize each option that appears there, to shorten it and tune it to our needs, since many people, including myself, complained that it's getting too long