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Well, from what I've found it's a feature of Chrome that allows automatic switching to legacy browsers, typically Internet Explorer, for a specific list of URLs, and where if you navigate back from that URL you can be set up to automatically go back to Chrome.
This appears to be something more typically used in large enterprise settings. They even mention the complete removal of IE this year and that the feature can now trigger Edge in IE Mode.
If you're not being "thrown out" of Chrome when that message appears then I would presume the instance of Chrome you're using is not completely configured for Chrome Legacy, which would be unsurprising for home users. The funny thing is that Chrome's own documentation says this is enabled after a certain Chrome version that I've now forgotten, but without a list of URLs on which to apply it there really is no effect.
I've never encountered this, but I also use Google Chrome browser very sparingly these days, preferring other Chromium-based browsers.
Brian - Windows 10, 64-Bit, Version 21H2, Build 19044
A man's worst difficulties begin when he is able to do as he likes.~ Thomas H. Huxley (1876)