Does this sentence make sense and sound natural for a friend?
(Talking about my schedule.)
Since it still unconfirmed, I will let you know later.

Question

Answer

It makes sense and would be good in formal contexts.

In informal contexts it would be better to say:

"I'll let you know when it's confirmed."
"When it's confirmed i'll let you know."
"I'll let you know later on when it's confirmed."
"It might be confirmed later, i'll let you know."

etc.

Answer

I'll let you know when it's confirmed 。