I agree with BasSchulte - Escaping is a kind of Encoding. Both are ways of translating between one symbol system and another. Escaping is a the subset of encodinng where the contents are enveloped within the target coding. It is irrelevant that escaping uses prefixes to tag metacharachters. The problem (over-encoding) can still exist in other contexts.
I like the idea of adding a 'canary' to detect over-coding, but I would prefer to use something more robust, like a CRC and I don't like the idea of using it to determine when to stop decoding.
In the multiple redirect situation described in the article, I would prefer to fix the root problem: the redirects should not have been re-escaping the original data. This canary solution just hides the problem.