But there really isn't any other solution. Yes, there will be an uncomfortable transition. Yes, it blows. But there isn't any other solution that is going to work other than deal with it and take the hits as they come. The software needs to be updated. The presumption that usernames are from some 7-bit ASCII subset is simply unreasonable. We'll be chasing bugs with these features for years. But that's not some sort of optional aspect that we can somehow work around. It's just what is coming down the pike. Better to grasp the nettle firmly [1] than shy away from it.
At least this transition can learn a lot from previous transitions, e.g., I would mandate something like NFKC normalization applied at the operating system level on the way in for API calls: https://en.wikipedia.org/wiki/Unicode_equivalence Unicode case folding decisions can also be made at that point. The point here not being these specific suggestions per se, but that previous efforts have already created a world where I can reference these problems and solutions with specific existing terminology and standards, rather than being the bleeding-edge code that is figuring this all out for the first time.
[1]: https://www.phrases.org.uk/meanings/grasp-the-nettle.html