The Language Workgroup discussed this pitch briefly and felt that it was unlikely to be accepted if brought to review in its current form. There was general agreement that it's probably appropriate to solve this problem with a source-level feature (to avoid the 'dialectization' issue raised by, e.g., compiler flags) and that deprecation warnings are a bit of an exception amongst Swift's warnings today in that there is no way to silence them when the user really does want to. However, there were also concerns about the potential downsides of a solution which allows for overly coarse-grained silencing of such warnings, some of which have been raised in this thread.
The Workgroup feels that this pitch would benefit from more discussion refocused on solutions which enable more narrowly tailored silencing mechanisms, similar to how other warnings are silenced.