To follow up on this:

1. There is usage inside Firefox itself! Firefox uses usage: "search", sensitivity: "base" equality to find duplicate entries in form autofill data.
2. The vast, vast majority of usage on the Web can be traced to a single PR: https://github.com/mapbox/mapbox-gl-js/pull/6270
3. There is also some other Open Source JS library (whose distinction is passing void 0 as the locale) that gets bundled in a bunch of places, but I haven't taken the time to identify which library it is. The sites don't look appy.
4. A product called Vanilla Forums uses it somehow always with "en" locale, i.e. the root search collation.
5. Other than those, there about 16 sites in HTTP Archive that match. 6 of those seem to be the same Polish food menu template vendor. The remaining 10 seem to be one-off uses each.

Both void 0 and "en" are fine in the sense that they map to the root search collation and don't involve root search data inheritance, which is the binary size problem.