That's my recollection as well although I can't prove it--and that's too long ago for Google Trends to be useful. In the US at least randonee also has a meaning distinct from backcountry cross country skiing in terms of equipment. I'm pretty sure I remember the outdoor group I'm a member of having backcountry (i.e. non-groomed trail) cross country skiing workshops going back to at least the 1990s.

The confusion, I think, comes from the conflation of two separate questions:

1. In ski area boundaries our outside (in-bounds, or "backcountry")

2. Downhill skiing, Randonee/Touring, or x-country (nothing to do with backcountry since you can do all of the above in bounds or in the backcountry).

Backcountry, at least where I grew up in the PNW in the 90s, only pertained to where you were skiing, not to equipment or technique used to ski it.

Ah, that's true -- "backcountry skis" tended in the past to refer to off-trail XC skis rather than the more expansive usage it sees today.