Monitoring referendum voting

Is there a way to monitor the % of online ALGO that has voted YES (i.e. upgraded their node)?

You can use this explorer:

Last 10,000 periods, there were 7424 votes YES.
But note that most likely many users that want to vote YES may not have yet updated.

@fabrice

Awesome, thank you. Is there any way of knowing how much ALGO this pertains to?

The above number means that over the 10,000 round periods, 7,424 block proposers voted YES.
The referendum is accepted if at least 9,000 block proposers over the corresponding 10,000 rounds period vote YES.