PoW on pubkeys, PoW on event ids and subscription ID length are different things for sure and unrelated.

PoW on events is what you do when you try many nonces on your event until you get an event ID with s minimum number of zero bits.

Subscription ID is how uniquely identify subscriptions over the same websocket connection.

Besides that, when querying for authors or event IDs there’s now a minimum length of 4 hex characters or two bytes. So you can search for the prefix of authors or events that start with zeroes but you must pass something like "0000".