The B-21 can most likely evade the low-band radar (search) of the S-400 / S-500 5th and maybe 6th Gen air defense weapons, but should it not, it still can evade the high ban (track) part. Our great B-52s can't. They will have to stand-off and use cruise missiles.
Why The Air Force Needs the Stealthy B-21 Bomber
Does America Really Need the B-21 Raider? The Russian state-backed media has for several years been claiming its advanced air defenses can track and destroy stealth aircraft, due to advanced computer processing, digital networking, and high-fidelity, longer-range radar systems.
Russian S-400 and S-500s
The Russian S-400s and S-500s, for example, reportedly reach longer ranges and detect aircraft on a wider range of frequencies, however, the ability to establish a moving target “track” and succeed in actually “hitting” or destroying a stealth aircraft is quite different than merely detecting that something is “there.”
Therefore, if advanced air defenses be able to see “stealth” aircraft such as bombers, does that make new platforms like the B-21 obsolete upon arrival?
The clear answer to this is likely no, for several reasons.
Aircraft like the B-21 are engineered to elude lower frequency surveillance radar which can determine if something is “there” or “in the area” as well as high-frequency engagement or targeting radar able to establish and maintain a target “track” on a stealth aircraft.
Finally, even if some kind of “track” is established, successfully “engaging” or destroying a target is entirely different and much more difficult.