Forward-compatibility testing for deprecated features is not fleshed out

There is a forward compatibility mode for the front-end, which is should disallow use of deprecated features. However, it is not fully fleshed out.

The issue names no files, tests, or specific deprecated features. Start by locating the front-end's forward-compatibility mode and its existing tests, then determine which deprecated-feature checks are missing. Done means the mode consistently rejects the intended deprecated features and has coverage demonstrating that behavior.

Written by the indexing model from the issue text.