Indeed all radars working in the V/UHF band (i.e. from 0.03 to 3GHz) are capable of detecting an F-22 or an F-35 (more info in this article). Thus all civilian air traffic control radars in the world detect, at least in part, stealthy aircraft.
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No aircraft can be invisible in every electromagnetic frequency, but the F-35 is designed to be so hard to detect in the frequencies used by targeting radars that an enemy would need to be nearly within visible range to even attempt a kill (very few enemies would be able to get that close without being shot down).
Restricted aircraftOperators of private or sensitive aircraft may request their tracking details be restricted or not displayed. Generally, aircraft requesting that they not be displayed on Flightradar24 are military aircraft. Some military aircraft, such as various transport aircraft, are visible.