Is it expected that LEGO will produce small (S or XS) PF motors? The existing LEGO PF motors are the M, L, and XL, making builders suspect that the S and even the XS names were left out and reserved intentionally for later designs. However, the M motor was released ten years ago, and we have not seen any smaller motors.

Are there any (pro or con) signs suggesting that those motors will or will not be released in the future?
Generally, speculative questions are considered off-topic, but in this case very little speculation is required to answer the question.

The clear answer is that S or XS PF motors will never be released.

The entire PF system is on its way out and is being replaced by the newer Powered Up system. For this reason, I would expect production on existing PF components to stop soon, and it seems highly unlikely that TLG would introduce new elements for PF at this point.

It is of course possible that they will include smaller motors as part of the Powered Up system at some point.