The splits should be practical. Domains are set so that way people can follow the questions which are related to them and their work. Modeling and simulation involves diffeqs (ODE/PDE/SDE/etc.), finite element, control theory, etc. If your question is about using those libraries in an engineering context, then you probably want to ask in a place where those developers and maintainers are actually looking/reading.
On that other hand, if the domain has a bunch of domain abstractions such that most users aren’t using the underlying core directly (for example, I know about the JuliaAstronomy that uses DifferentialEquations.jl/OrdinaryDiffEq.jl underneath), then the questions should be going to the domain experts and their package developers and not necessarily everyone who uses a diffeq or finite element package. So it makes sense for astro to have a domain since a lot of people doing astronomy, while they may be solving simulations, they may not be actively interacting with the core numerical simulation libraries but a domain-specific abstraction on top of it.
But I still think (as I noted from the very beginning of the Discourse) that modeling/simulation should just be merged with numerics so that way it’s just the domain of numerical methods for scientific computing. Then any application is in some other domain.