Above 18,000 feet is class A airspace (at least here in Canada), and Class A requires you to be on an IFR flight plan.
Not exactly true. From the Aeronautical Information Manual:
3[ch8722]2[ch8722]2. Class A Airspace
a. Definition. Generally, that airspace from
18,000 feet MSL up to and including FL 600,
including the airspace overlying the waters within
12 nautical miles of the coast of the 48 contiguous
States and Alaska; and designated international
airspace beyond 12 nautical miles of the coast of the
48 contiguous States and Alaska within areas of
domestic radio navigational signal or ATC radar
coverage, and within which domestic procedures are
applied.
b. Operating Rules and Pilot/Equipment
Requirements.
Unless otherwise authorized, all
persons must operate their aircraft under IFR. (See
14 CFR Section 71.33 and 14 CFR Section 91.167
through 14 CFR Section 91.193.)