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It's definitely true that case-folding is generalisable to more generous equivalence notions - for example, more or less whitespace isn't a noticeable difference that people typically want two files for. People casually using their phones want to treat the filesystem as a search tool where fuzzy matches (across different directories and names) are reasonable responses to file system queries. There have been many attempts at it like WinFS, but that's a significant increase in scope from case-folding.

One incontrovertible justification for case-folding is to support Samba shares efficiently. Samba has to implement case-folding to clients and if it is not done in the filesystem datastructures you have to read in many directory entries to figure out if one would match with case-folding.

There is plenty of MS Windows and MacOS software that just expects case-folding from the operating system.

The article is old and this was released in 5.2. It can be turned on per (empty) directory with chattr https://kernelnewbies.org/Linux_5.2#Optional_support_for_cas...