1. In void M([DisallowNull]string x) the attribute is redundant.
   Same in void M<T>([DisallowNull]T t) where T : class or void M<T>([DisallowNull]T t) where T : struct.

2. [MaybeNull]int or [MaybeNull]int?.

3. void M([MaybeNull] string s) (the attribute will affect the state of the argument, but why would anyone want that?)

4. void M([DoesNotReturnIf(true)] out bool)

5. [MaybeNull, NotNullWhen(false)] (should work per Tweak interaction of outbound annotations ([MaybeNull, NotNullWhen(false)]) #36410)

6. MaybeNull and NotNull conflict

7. MaybeNullWhen(b) and NotNullWhen(b) (same b) conflict

8. int M([MaybeNullWhen(true) string x) should warn because return type is not bool

9. [DisallowNull] string? Property { get { ... } } or [MaybeNull] string Property { set { ... } } or [DisallowNull] readonly string? field = ...;

10. using [DisallowNull] or other attributes outside of a nullable annotations context (Usage of flow analysis attributes should warn outside of annotation context #36588)

11. using [NotNullIfNotNull(parameter)] for a parameter that doesn't exist

Note that some of those may be useful on ref parameters: void M([DisallowNull]ref string? s) or void M([MaybeNull]ref string s).
Also, the conditional attributes seem useful on non-generic types: [MaybeNullWhen(true)]string s or [NotNullWhen(true)]string? s.

Relates to #36009 (Should [DisallowNull] apply to value types or nullable value types?)

Relates to PR #35955