Relativity hasn’t replaced Newtonian Mechanics for 99.9% of the current use cases it combined with Kepler’s Law of Motion is still used even in astrophysics and astronomy at large the only points at which Newtonian physics really breaks down is in the presence of strong fields and we are talking about black holes type of strong fields or when you try to calculate things on the scale of the observable universe.

Newtonian mechanics is still very accurate and very useful if it wasn’t it wouldn’t be thought on an undergraduate and even graduate levels, ironically it’s also what we’ve used to figure out things like oh shit were missing a lot of matter in the universe.

If it wasn’t so darn close to being accurate in pretty much every scenario things like MOND wouldn’t have a reason to exist either because for quite a while in fact until the confirmation of gravitational waves there was enough wrong with general relativity to cast doubt on it too, however even with gravitational waves killing most of the modified Newtonian candidates Newtonian mechanics is a darn useful tool still and it’s usefulness would likely never go away.