Well to be more specific, the Turing Test is designed to test a computer for the presence of artificial intelligence. That it can be used to tell the difference between a human and a bot merely exploits our current lack of true artificial intelligence. Something which is steadily changing.
The examples wouldn't work for many channels.
- The mixed-up letters would most likely defeat people who aren't particularly familiar with the language the test is made in. Because many IRC channel's userbases include multiple countries that's not very practical.
- The same goes to the one using a common saying.
- To some extent the same goes for the word-link one too.
A 'simple maths' test could probably be beaten by a bot.
In all it would be a hell of a lot simpler to just use a network which isn't plagued by spam. There are plenty about.
