§128

Theses:

(1) According to the Oxford dictionary on my kindle a thesis is, "a statement or theory that is put forward as a premise to be maintained or proved".
(2) According to PMS Hacker, the word 'thesis' might be used to refer to, "...debatable claims about the nature of things such as are advanced in the Tractatus" or it might be used to refer to,
(3) ...grammmatical propositions.

Why are there no debatable theses in philosophy?

(1) - If a thesis is a statement put forward to be proved (empirically) then philosophy (as Wittgenstein conceives it) does not contain theses at all, let alone debatable ones. According to Wittgenstein philosophy is not an empirical discipline - it doesn't set out to make discoveries about the world or to perform experiments but to dissolve conceptual confusions.
(2) - If a thesis is something like the claims of the Tractatus (e.g. reality consists of facts not things), which are supposed to be pronouncements about essences, then there cannot be any theses because there are no objective, language-independent essences.
(3) - If a thesis is a grammatical statement (e.g. something cannot be red and green all over) then it is quite unlike a (debatable) empirical claim. Empirical claims might be true or false. Their denials make sense. But the denial of a grammatical 'claim' is nonsense. The response to nonsense is to 'assemble reminders' - not to assemble evidence. - So grammatical claims are not debatable as empirical claims are.

So - if 'thesis' means one of the above ((1), (2), (3)) then there are no debatable thesis in philosophy.