low altitude penetration is still a niche served by the B1-B Lancer in the USAF. Penetration cannot occur at stand off range using stand off munitions sometimes you have to kick the door down and see what pops up before you can kill it.
I think low altitude ingress will over time be the sole preserve of unmanned platforms. Today, we have loitering munitions like the Tomahawk for pop up threats. This new Tomahawk capability was tested very effectively in Libya, fly in low below 50 or 100 ft hug the terrain and you are practically invisible. Use your conformal interferometer arrays to geolocate and evade or destroy threats.
What are my thoughts on J-20's ATG capabilities? Not a clue, when I saw a picture of the J-20 for the first time, I thought it was purpose built for defensive counter air, I think the Pentagon came to the same conclusion and have since then introduced platforms and tactics to mitigate the risk to slow air in or near the theatre. I don't think the J-20 was designed for SAM hunting or close air support. Strategic deep strikes sure - but, I'm not convinced the J-20 has long legs.
The B-1B seems to be 1 huge target to fly in and out of danger zone safely at such low terrain masking altitude today given the deadly new generation SAM system that has ability to detect, track, acquire lock and shoot in short time? Better deploy numbers of smaller lower cost fighters such as F-16 for search and destroy sorties while F-15E on second line clearing the path out of enemy SAMs and AAAs paving way for fighters and bombers on strike package for enemy structures. Over Afghanistan, the US fighters didn't really fly low to release JDAM, Paveways as newer FLIR targeting pods such as SNIPER, ATFLIR, Litening are superior than the old LANTIRN.
Russian Air Force Su-30SM and Russian Navy Su-34 shot down over Ukraine clearly tells the deadly effective SAM system as even MANPADS such as Starstreak could accelerate to Mach 4 splashing the Mi-28 before it could even react for defensive countermeasures and maneuver at range of less than 2Nm. Most of Russian air to ground missiles Kh-25, Kh-31, Kh-59 seekers relying on active radar guidance that can't distinguish friendly and enemy targets if enemies are hiding in populated cities. Their laser guided missiles and bombs have shorter effective range than US counterpart due to poorer targeting system requiring fighters to fly closer in order to designate target on the blurry TV on MFD or through HUD. Looking at big heavy Kh-29T/L vs AGM-65E/F/D/G, the Mavericks have longer effective range and accuracy whereas Kh-29T/L relying on huge warhead to blast anything within that radius if it didn't land exactly on target. Unguided rockets and bombs are totally obsolete to be carried on fighters nowadays looking at how hopeless the 4th generations Russian fighter bombers are over Ukraine.
PLAAF fighters been relying mostly on similar air to ground ordnance until after they managed to reverse engineer LANTIRN and come up with more laser guided missiles and bombs for their newer fighters such as J-16, J-10B/C and J-11D. Guess they should be able to fit FLIR targeting pod for J-20 later as some expert in forum said that the angular spade glass beneath the J-20 fore bottom fuselage just after radome (that looks like F-35 built in EOTS FLIR + IRST pod ) is only IRST without the FLIR pod. Everyone including me thought the J-20 has EOTS FLIR + IRST like F-35 until few guys in this forum told me that is just IRST, not for designating ground targets. If J-20A/B aren't equipped with FLIR, then its air to ground capability is just limited to launching radar guided missiles or passive guided missiles or GPS guided missiles/bombs. Apart from Blue Arrow series laser guided air to ground missiles and LS series laser guided bombs, I don't see any other tactical air to ground missiles that don't need to turn radar.