For 1.5 I think the main issue is not removing functions from the core - because they are rarely used - but differences in the newer PHP versions.

For 2.0 also the biggest issue is upgrading templates to the newer Smarty syntax. 

I hope that 2.0 will have a modules testing framework, so it will be easier to find if a module breaks with newer versions of CMS or PHP.

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About that post from 2012 - what I really know is that probably we still will not have the system module rewritten fully with IPF. Sadly I don't have so much time to such big changes.