1. It is easier to keep the description of significant changes closer to the code, hopefully all plugins will have these instructions for upgrades in the future. The problem with docs is that they get easily out-of-sync. That file is supposed to be a sort of checklist with links to docs pages with the actual description of changes.
2. This in not about upgrade from 1.9->2.0 (which is huge I agree), but 2.1->2.2 because upgrade from older versions will not be probably supported. Each version brings incompatible changes and requires update of code, I personally prefer exceptions that force devs to upgrade code instead of developer debug messages that are being often ignored.
If you value your reputation please test all your plugins in each new major version, there is no warranty it will work. The 2.1 release was a bit special (few changes) because it was under active development for a short time only.
Please do not forget this is also a potential security issue, until we remove the debugging messages we did not solve this problem. This is the reason why I would vote for removing of the array debugging with fallbacks in optional_param() and required_param() before the Moodle 2.2 release, in any case debugging should be removed from 2.3.
General developer forum
HEADS UP: changes in parameters cleaning
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