Factorio Forums

Per this thread, fluidbox filters on pumps to allow systems to contain multiple fluids are not an intended behaviour.

Therefore, pump fluidbox filters should not be settable by script or by prototype, to prevent modded filtering pumps from being created which would allow this.

The simple fluid mixing was implemented specifically so that mixed fluids aren't a problem anymore so we don't have to hunt down every single corner case that creates mixed fluids.
Since pumps with filtered fluid boxes by themselves don't create fluid mixing or crash the game, I don't see any reason to remove this modding feature.

I think you may be mistaking "we don't want to support/encourage this" (simple fluid mixing) with "nobody should be able to do this" (previous fluid mixing prevention).