The semicircular rule applies outside controlled airspace, and these days is pretty much irrelevant.
... Except in the UK where the Quadrantal Rule is applied for IFR flights
Actually
Instrument Flight Rules
32.—...
(2) For flights outside controlled airspace rules 33 and 34 shall be the Instrument Flight Rules.
...
Quadrantal rule and semi-circular rule
34.—(1) Subject to paragraphs (2) and (3), an aircraft in level flight above 3,000 feet above mean sea level or above the appropriate transition altitude, whichever is the higher, shall be flown at a level appropriate to its magnetic track, in accordance with Table 1 or Table 2, as appropriate.
...
Table 1—Flights At Levels Below 19,500 Feet
...
[Resetting Table 2 to allow posting here]
Magnetic Track Cruising Level
Less than 180°: 21,000 feet, 23,000 feet, 25,000 feet, 27,000 feet etc.
180° but less than 360°: 20,000 feet, 22,000 feet, 24,000 feet, 26,000 feet etc.
So
the semi-circular rule
does exist in the UK for flights outside controlled airspace, it's just that we don't have any airspace in which it
can apply. But the convention identical in detail to the semi-circular rule but not in fact a flight rule tends to be applied as a preference within controlled airspace.