Genuinely interested in how modern radar would get around the line of sight issues? Doesn't line of sight mean that yes, you absolutely can still fly under radar?
LoS is different to the original concept of "flying below radar". The original concept was based on the problem of ground scatter - below a certain altitude the radar beam is hitting the ground and producing spurious returns which mean they swamp out any interesting signals.
But modern radar is much more sensitive, and the noise-handling algorithms better - basically it's much more able to distinguish "I hit some trees with my beam" versus "metal". Combine that with modern transmitters which can also produce tighter beams and all the other electronic goodness, and the net effect is that you can scan just as low as you can see in all directions and filter out everything which is boring (i.e. the ground doesn't actually move very much, so really it's a fixed background on your radio image).
The other element of this, is AWACS: AWACS radars are higher, and look down. So not only do they go much further, but there is no "below radar" with them - they can be operated from much further away, and have all the same advantages (i.e. much better signal return discrimination). An AWACS will see you on radar long before you're in weapons range (hence the body of them orbiting near the Ukraine border these days).
The final element is that "flying below radar" was also just plain never that effective. You can test this by asking how often it's actually been done, if it's so effective. If it worked all the time, then that's what military's would train to do. Instead the only real advantage it provides is it reduces light of sight, and that's not uniformly applicable - i.e. a radar on elevated terrain would be able to spot aircraft making low approaches around it because it can just look down and pick out the airplane-like returns and ignore the ground scatter - and said radar can be far behind the lines.
Modern radar can detect and track low flying aircraft just fine.