[sillybov3456]

I think based on the parent posts choice to go with the "who is your customer" definition and the fact that we intuitively think of RabbitMQ as some sort of systems program, it probably fits the modern definition of systems program that we all kind of agree with but do not necessarily put into words.

I don't think RabbitMQ being written in a "non systems language" is relevant. Its a systems program even if it was written in shell (if it was it might be a bad systems program, but a systems program nonetheless because of it's intended customers and use case).

Which brings us to browsers, which are not systems programs (and I think we tend to not think of them as such) almost entirely due to their primary customers not being programmers. This is despite the fact that they fit the "complexity/size and close to the metal" definitions a lot better than many other programs that are commonly considered to be systems level.