Earlier quoted context omitted.
You could manage that with hashed emails by separately storing the case of letters in the user part, without storing which letters it's referring to. You could then apply that case to the user-provided email during checkout. (You could probably actually do a pretty good job in most cases just by bruteforcing the possible combinations and seeing which one matches the hash, but the worse-case CPU cost would be bad for…
This was proposed in another comment as well, and it’s a neat idea that unfortunately becomes fragile in the face of UTF-8 local parts, since the Unicode folding standards change over time.
Fair point, though you could probably store all non-ASCII characters in plaintext and still get most of the benefit. I suspect UTF-8 in emails is rare overall, at least for sites that don't have certain region-limited audiences.