Interesting, thanks. I have virtually zero JS experience so didn't know that, and were I to try and pick it up this would've been a surprise to me - I naively assumed it would use the same terminology as the RFC and the libraries in languages with which I am familiar (rust, golang, python, perl). A quick look for the URI data type in a handful of other languages shows they also use "scheme". JS appears to be the outlier here.
In particular, the scheme doesn't necessarily refer to a protocol (e.g. application-specific schemes like `chrome:` that don't involve network access at all).