Why are SIGUSR 1 and 2 deprecated?

The example config (as well as i3's default config) uses SIGUSR1 (signal 10) to signal volume changes. Unfortunately, when I tried this, it silently failed. Only when running i3blocks from the command line did I see the problem:
```
<3>SIGUSR{1,2} are deprecated, ignoring.
```
There is no mention of this in the i3blocks documentation, and the example config still uses signal 10.