250 knots below 10,000 feet is mandatory in all airspace classifications which rely on "see and avoid" to separate the traffic permitted to fly within them. That's ICAO class C, D, E, F & G. This speed limit is a mandatory requirement of the airspace and may not be relaxed by ATC.
That's correct, in a roundabout way ("C" does not wholly rely on "See and Avoid"). In ICAO C, IFR has no speed limits. Only VFR has a 250k < 10,000' limit.