It is completely and utterly irrelevant whether it is always energised or not. The fact is that people know that it is a line conductor and therefore at a potential of 230V to Earth. They also know not to connect it to the system neutral which is at Earth potential.
If you open up a switch and see either red & black or brown & blue going to the two terminals of a single-pole switch, you should know that both are live (or potentially so, no pun intended) and that the black or blue is not a neutral (just the same as if you see black & white on a single-pole switch here you know the white isn't neutral).
Surely the purpose of color coding conductors is to assist in identification? If it's clear what purpose the wire is serving, then the job is done. It could be argued that using twin brown rather than brown & blue on a simple switch drop actually hinders identification, since once one end is connected you can't easily tell which wire is which at the other end. With brown & blue (whether or not the blue is sleeved brown), it's easy. Of course, in this case it's hardly a big issue since you can easily test and if you're just connecting to a single 1-way switch it doesn't matter anyway, but nevertheless, if one insists on following that "preferably" clause, it actually makes identification harder.
On a related switch drop issue, under the "old" system, with red/yellow/blue (or the older red/white/blue) to a 2-way switch I'd say that the yellow (or white) & blue were probably
not sleeved with red more often than they were (at least in residential systems). Did/does it matter? You know that in an installation fed from a 2-wire single-phase supply they can't be other phases.
As Winston noted, in countries where singles run in conduit are common even for residential work, it's also very common to use different colors for 1-way switch returns, 2-way travelers, etc. And why not if it helps to identify different conductors? Isn't that the whole purpose of color-coding them in the first place?