It could be many things, as there is no validated reason as to why these stations excist, there are plenty more of these stations as well, there is so many differant theories as to why it has stopped and what it does, such as:

Agents in differant countries who must remain unidentified, the signal broadcasting the voice codes could give directions etcetera, if the station stops it could possibly mean that the agents or whomever should evacuate back ASAP - but this is me just giving an example.

Differant stations stopping could mean differant warnings to the military, it could be an early indetification to a disaster, it could mean that the intended listeners must switch to a differant broadcast or follow next instructions, without understanding how it was designed or having anybody confirm what it means there is not much we can do apart from research and give opinions...

Either way I find it spooky that it has stopped after so many years of 24/7 broadcasting. It obviously emans something important otherwise it would not have broadcasted so much in my opnion.

(Perhaps it is worth listing other radio stations and noting down whether they have changed tones or even stopped broadcasting as well).