FWIW I decided against using IEnumerable since there are still a lot of performance problems there (and the purpose of this change is 99% around performance). They are being largely addressed by .NET 10 apparently. This does make implementing a collection that allocates once and never again notably harder. I still don't think it's bad, you just can't yield return the implementation away.

Besides, most users should be able to get away with ArraySegment, possibly with ArrayPool in the background. Still not sure of the status of ArraySegment on certain NET Framework versions, but I think it's kosher on everything .NET Standard 2.1 and later.

Come to think of it, replacing most of our explicit array constructions with ArrayPool and ArraySegment would be a decent project to consider (provided it's possible on all of our targets).