In Phabricator, there is the tag "Technical Debt", column "Deprecate / Remove" were examples can be found. I first thought about adding the link on the bottom of the page, but then I saw there is nothing about "centralising" the bugs related to deprecation, so I have been bold in adding it: only recommanding to tag bugs in order to find them more easily, not something like "all deprecation MUST/SHOULD be tracked in the bug tracker", I guess it would be too strict. If I have been too bold, feel free to remove this sentence.