The Ukrainian air force with mostly su-24's and su-27's should not exist or even be able to operate against Russia's supposed air defense capabilities(s-400, s-300, Tor, manpads) but war is almost always a battle of information and the stealth and network capabilities the F-35 has puts it a league of its own. One which only the f-22 could transcend but the raw number of F-35's being pumped out negates that advantage to a degree
one side has untouchable radars and AWACS.
At what altitude? Earth isn't flat, so even at 20km and superb electronics you don't see what happens near the ground.
Though probably someone already published their routes, but I too lazy to check, honestly. Still my point stands - As can be downed (and everyone would clap, especially in the US) while Es "NO YOU DONT THATS ARTICLE 5"
Over distances of 600km the vertical offset due to earth's curvature is about 20km. If it flies at it's operational ceiling of 14km, it can offset most of that but wouldn't have direct line of sight on planes at extreme range flying below 6km. If it has over the horizon capability though, which honestly seems highly likely for a recently updated system, then it should be able to compensate for the remaining shadow with some loss of resolution. Which I'm guessing is why it's estimated range is considered 600km. I'm sure the military analysts making those estimates know about these effects.
Anyway, southern Ukraine can be directly radar imaged from over Crimea so it's really not an issue.
In the specific case of the A-10, it was designed for a world that had extremely primitive sensor systems that couldn’t even reliably lock onto a plane when looking straight at it. We aren’t in that world anymore. Not only is the terminal guidance sophisticated and reliable but the warheads are designed specifically to destroy aircraft like the A-10. Everyone has had 50 years to work on that problem.
Modern Russian and Chinese systems effortlessly deal with an A-10; it lacks the power systems to support modern counter-measures against modern systems. It isn’t upgradable like that because that wasn’t a design objective.
Modern military aircraft like the F-35 have critical combat roles that don’t involve dogfighting. Their sensor suites and processing capabilities are at least as important as any other particular feature, and other aircraft don’t have that. Their systems can measure and exploit their environment to an astonishing degree.