There's no 'S-300' as such, there are sets of fire control, target acquisition and tracking radars, and various types of missiles, each of which can be upgraded, and mixed and matched to some degree, with some combinations being up to the S-300 standard or better.

The closest thing to a standardized variant is the one installed on ships.

It's a crazy variety of hardware out therem and one of the most dangerous things about SAMs, that a lot of the old Soviet missile stock is passively guided, so pairing a decades old missile sitting in storage with a state of the art radar makes it relevant even today.

Air defense works in layers where each layer often covers for another. S300 is good, but it's just one piece of a useful anti air defense strategy.

S300 is very good AA, but in practice modern SEAD with a sizeable number of planes can outrange them and they're not great at protecting themselves. We saw this in India-Pakistan and seeing this again in Iran-USA. You can see more of a stale mate when they aren't getting outranged in Ukraine-Russia.