While you obviously have access to a dictionary, and have correctly quoted a reasonable definition of the word “niche”, you don’t seem to be applying it in the correct way.
Is the Window’s OS niche ? I would say no, given it’s currently one of the most used consumer OS by far. The built-in file manager, File Explorer (which is basic, but provide’s standard, most used functionality), has probably been designed to cater for the basic, non-niche usage, but gets the job done in most cases.
Given that, why does this basic, non-niche, built-in tool, support ADS by default when copying a file ? Could it be because it actually isn’t that niche (end-user’s don’t need to be aware of it to be non-niche) ?
Could implement it myself and contribute a patch but, at the same time, it should be a default feature of a backup app (like-wise with other certain file system attributes which cannot be re-created without the source), and ultimately, the devleoper is being paid to do it.
Implementation should actually be relatively trivial given that ADS’s are simply other file’s that are normally hidden from the end-user, and the OS natively supports them, so you don’t have to do some wizardry to access them. One quick Cross-platform/portable suggestion would be to store them in a ZIP (.7z would be better, which should also work with cloud storage + de-dupe). During backup it would check if source/repo is NTFS - if so, then check each file/folder for ADS’s, and back them up in ZIP/7z. On restore, if dest is NTFS, just restore ADS’s (make optional), otherwise decide whether to resore in source’s extended attributes.
The term “Backup” is subjective, but if potentially important data is lost, then it isn’t a true backup solution - why backup with a tool when you lose information/data which cannot be re-created without source ?
As with everything, everyone has their own opinion though.