let t1 = "" :> IEquatable<string>
// Nullness warning: A non-nullable 'string' was expected but this expression is nullable. Consider either changing the target to also be nullable, or use pattern matching to safely handle the null case of this expression.

VS 17.14.6

Misc notes:

• The IEquatable interface unfortunately marks arguments to Equals as nullable generic mathematics interfaces should use nullable reference types runtime#73855 (comment), a wart which F# seems to squash, allowing IEquatable<'T> to be defined with Equals(y: 'T) = .... But that doesn't seem directly relevant to the above issue.
• "" :> IEquatable<string | null> doesn't give a warning
• A workaround is to define a wrapper [<Struct>] type StringWrapper(s: string) = member _.Value = s.
• This applies to types where null is "allowed":

[<AllowNullLiteral>]
type NullAllowed() =
    interface IEquatable<NullAllowed> with member _.Equals(y: NullAllowed) = true
let _ = NullAllowed() :> IEquatable<NullAllowed>
// Nullness warning: The types 'IEquatable<NullStringWrapper>' and 'IEquatable<NullStringWrapper> | null' do not have compatible nullability.