Learn Italian: Quello che sappiamo di questo rito è solo una tradizione orale. - What we know about this rite is only an oral tradition.

When 'what' means 'the thing that' or 'that which' in a statement, Italian uses quello che or ciò che. You can only use cosa (or che cosa) for direct and indirect questions, like 'Cosa sappiamo?' (What do we know?).

Could I use ciò che instead of quello che here?

Yes, ciò che is completely interchangeable with quello che. Quello che is more common in spoken and everyday Italian, while ciò che has a slightly more formal or literary tone, which would fit perfectly when discussing a rito (rite) or tradizione.

Why does the sentence use di questo rito instead of su questo rito for 'about this rite'?

Both sapere di and sapere su mean 'to know about'. Sapere di is extremely common for expressing general knowledge or awareness of a topic (knowing of it). Sapere su is also correct but tends to imply knowing specific facts or details on a subject.

How does this static sentence relate to the lesson's focus on continuous historical actions?

This sentence uses the simple present (è solo una tradizione) to state a static fact, setting up the context. To express the historical continuity you are learning, you could expand this sentence by adding a phrase like una tradizione che ci viene tramandata (a tradition that is being handed down to us).