As per my comments in #1162:

• Deprecated features should be a warning
• Removed features should be an error
• New features should be an error
• For the few things which don't fall in the above categories: whether it is an error/warning should depend on how PHP handles the issue, i.e. does it cause a PHP parse/fatal error or a notice or warning to be thrown ?

Similarly for the error codes:

• If a sniff throws both errors and warnings or is expected to - at some point in the future -, the error codes should be distinct or at least be set up to allow for them to be distinct. Think: Deprecated for deprecation warnings, Removed for removed feature errors. Not: Found.