Interesting! do you have any links talking about that standard? I'm super interested in Julia and this seems like a good opportunity to learn something I've been missing so far.
I'm not sure if/where it's formalized, but it's just generally something that's been enforced throughout Julia's Base, along with many of the package organizations (like SciML among others). It's something that would be mentioned at code review time by most contributors. It's why you don't see unicode keyword arguments. There's a lot of reasons. I think the best one is that you want the API to be compatible with old terminals you tend to get on HPCs which do not tend to support unicode. We should probably make it a part of the standard formatter rules or something at this point.
This was added recently, and for some reason the PR (1840) didn't fix the docs, which is bad. The Optimisers.jl version has an explanation: https://fluxml.ai/Optimisers.jl/dev/api/#Optimisers.RMSProp