How "doubtLESS" and "NO doubt" lost their literal sense, and now suggest probability or concession?

Doubtless: is used to mean that you are certain that something will happen or is true

They will doubtless protest, but there's nothing they can do.

Doubtless you have heard the news already.

Cambridge dictionary

In these examples, doubtless appears as an adverbial qualifier based on opinion rather than fact. The assertion appears certain, but is (at least in principle) left open to negation.

This usage contrasts with related statements such as “There is no doubt that they will protest” or “There is no doubt that you have heard the news”, which are statements of certainty, presumably based on some proven or incontrovertible facts.

This slight shift of meaning between the different constructions is consistent with the flexibility of language and the development of idiom.

There is no doubt that you have read this answer. No doubt the answer helps with your question.