1- if you wanted to find a system not properly named, you had to make sure to have read the relevant .asd file beforehand.
For subsystems that's a perfectly reasonable case.
2- if someone defined two systems with the same name in two different files, then ASDF 2.014 could loop infinitely, and even 2.016 or later might survive but behave in unstable way depending on how changes may cause one system to be loaded instead of the other, then reloaded, etc.
If that were bugs which were fixable, then bringing up old ASDF releases doesn't make a compelling argument for prohibiting behavior.
ASDF is still backward-compatible. You can hush the warning if you want—though it importantly tells you which systems could use a new maintainer.
That is a very unfair statement. I'm not even sure how to comment it. I'll only note that not willing to change a code without a good reason (because issueing this warning is a fad) is conservative in a good sense of this word.
-- Daniel