I propose a new standard attribute, [[discouraged]], and a new macro, #strict, to indicating that the name or entity declared with this attribute is discouraged from use, and giving an alternative.

[[discouraged( string-literal )]]
string-literal - an unevaluated string literal that could be used to explain the rationale for discouragement and to suggest a replacing entity

macro #strict
#strict can appear anywhere in the file, and once the file contains this macro, the compiler will generate warnings for all uses of names and syntax that are marked as discouraged, and give the reason.

Full proposal written with bikeshed here: https://api.csswg.org/bikeshed/?url=https://storage.nykz.org/proposals/keeping-cxx-young.bs&force=1.