Yes its an open secret the Israeli F-35i's have been flying circles around the S-400 and S-300 systems in Syria, which its pretty much designed to do. They have even flown over Damascus and the Syrian's have been given the go ahead to shoot but havn't been able to lock on let alone get a target lock for the actual missile. Probably why the Israeli's havn't bothered taking out the radars.
The B-21 Bomber, which should be flying in 2021, will have the RCS of a mosquito and it will be a truly high altitude Bomber which will be making attack runs at 60,000'. The B2 was originally going to be made to fly that high but at the time there was a concern technology would force it to be necessary to have a low Alt capability so it's very rarely flown over 40,000'. Of course the higher you are the harder it is to be detected and/or target locked by the missile itself. The Bombers also carry a suite of defensive ECM.
Advanced low frequency radars are indeed capable of seeing stealth is in the area but their range is short and are pretty useless in acquiring a target lock for the missile itself. So when the Russian's brag about their new "Sunflower" Low Freq system take it with a grain of salt, and a very large one. The Australians have had such a system in place for many years, it was designed to monitor their huge coastlines and not for air defense but yes under optimal conditions it can tell you if a stealth aircraft is in the area but at best it can give you a few minutes to maybe get to a bomb shelter.
Also these Low Freq systems, which are unable to get target locks on the airplanes, are big fat targets for HARM missiles just like the high freq systems are. They are also highly affected by weather and light conditions. For instance at sunrise and sun down they are virtually blind and unless someone can re-write the laws of physics stealth will continue to be very effective for many years to come.