linter: unknown ESLint rules (e.g. custom rules) are reported as unused directives instead of being ignored?

[Boshen]

Is it by design or a bug that unknown ESLint rules (e.g. custom rules) are reported as unused directives instead of being ignored?

Originally posted by @donaldpipowitch in #9223 (comment)

[taearls]

Opened a PR for this issue here.