It depends on what you choose as your choice of technology, if you choose web components then you really can only really offer that same experience to users that have a browser that supports that API natively (without polyfills) which old Edge did not do.
My understanding of this situation with yt was that our server side detection code was wrong based on an update in edge (or our UA management), and we don't do feature detection in the client because it is too slow... So we send people to the older version.
Interpreting user agent strings is what amateur programmers do. I don't generally expect high standards from Google engineers (a whole other argument I won't entertain here) but that's still pretty tragic for a top-five website.
And besides, your claim is doubtful at best since Chromium Edge doesn't share any User Agent string elements with previous EdgeHTML versions. Your YT developers would have needed to be intentionally malicious to match "Edg/" as a trigger to downgrade the user experience.
You appear to have somehow missed the extremely loud chorus of "we hate it, change it back" that happen every time any web app gets a new design. See also the saga of Instagram on Android.
It depends on what you choose as your choice of technology, if you choose web components then you really can only really offer that same experience to users that have a browser that supports that API natively (without polyfills) which old Edge did not do.
My understanding of this situation with yt was that our server side detection code was wrong based on an update in edge (or our UA management), and we don't do feature detection in the client because it is too slow... So we send people to the older version.