My question is: are these fifth-character suffixes compulsory, or are they voluntary? For example, if I am looking at an arbitrary ticker, can I assume that it is an ADR if and only if the ticker is a five-letter ticker ending in "Y"? Similarly, if I am looking at an arbitrary ticker, can I assume that it is a warrant if and only if the ticker is a five-letter ticker ending in "W"?
A stock symbol with a fifth letter indicates that there are additional circumstances. This additional letter is an identifier that tells you something else about the security (class, warrant, right, voting, bankruptcy, etc.).
While these identifiers are FINRA based, individual exchanges have leeway in their 'symbology'. It eludes me as to why these small differences would be of importance.