There are no known users of the return value of OutputPage::adaptCdnTTL() according to https://codesearch.wmflabs.org/search/?q=adaptCdnTTL&i=nope&files=&a...
But our all-seeing eye does not see everything. Be it known that https://gerrit.wikimedia.org/r/450075, just merged, removed the return value for OutputPage::adaptCdnTTL() and this is technically a breaking change.
The value formerly returned by adaptCdnTTL() was arguably misleading: callers would probably expect it to return mCdnMaxage, but instead it (formerly) returned mCdnMaxageLimit. Since the return value was easily misunderstoood *and* it was unused, we've decided to remove it.
@simetrical (Aryeh) wrote the patch and should get the credit (the test coverage for OutputPage was also significantly increased! Yay, Aryeh!). Krinkle and I thought it was worth a C+2, so if in fact OutputPage::adaptCdnTTL() was vital to your way of life and we've ruined it forever, we should get the blame. --scott