[miki123211]

Probably more convoluted protocols, because there are always things that you do accept and that can be used to negotiate protocol extensions.

Imagine a protocol where both sides have to speak JSON with a rigidly-defined structure, and none of the sides is allowed to ask whether the other supports any extension. Such a protocol looks impossible to extend, but that is not the case, you can indicate that you speak a "relaxed" version of that protocol by e.g. following your first left brace by a predefined, large number of whitespace characters. If you see a client doing this, you know they won't drop the connection if you include a supported_extensions field, and you're still able to speak the rigid version to strict clients.

[quesera]

This made me laugh, because it's even more terrible than the most ridiculous chicanery we had to vomit into HTML and CSS over the years (most of which was the fault of MSIE6).