I just looked into this.
Interestingly, the rule incorrectly false negatives on

type T = { a: { b: { c: string } } }
declare const x: T | null;
x?.a?.b?.c;

why? Because it inspects the type of the expression being optional chained.
The type of x?.a is T['a'] | undefined.
So as far as the current logic is concerned, that's a valid optional chain target.

It should be possible to work around this though.

Hy @bradzacher I want to confirm about the issue that we have to make these conditions valid optional chaining.

type T = { a: { b: { c: string } } }
declare const x: T | null;
x?.a?.b?.c;

correct.

declare const x: { a: { b: { c: string } } } | null;
    x?.a?.b?.c;
//   ^^ valid
//      ^^ ^^ both error

declare const y: { a: { b?: { c: string } } } | null;
    y?.a?.b?.c;
//   ^^    ^^ valid
//      ^^ error