13 but not if it is unintentional, but God allows it to happen; then I will appoint you a place where he shall flee. Exodus 21:13WEB
This passage contains a positive command or statute, directed toward Israel: "If a someone unintentionally kills a man, I will designate for you the place where he should flee".
Cities of refuge are an essential component of Biblical Civil Law pertaining to manslaughter, and appear therefore to have abiding necessity in the context of modern Biblical Government. The details of how this works in the modern context are derived in a more complex manner, since (among other things) the cities themselves were to be governed by Levites, and the Levitical order is certainly passed away.
Notes on Interpretation/Application
Coherence of the Law
It's significant that this passage about the cities of refuge is given in Exodus, but the cities themselves, and their purpose, are not detailed until Numbers. This kind of forward reference evidences that Biblical Law is a coherent set from a single author, who planned or knew the entire law in advance of issuing its statutes; it is not a series of disjointed, loosely-related books written over a long period of time.