Most corp+school firewalls block https or make the user add a MITM cert (well, it's managed via active directory for company machines). Rarely do these firewalls allow websockets.

I don't think that any company can still block https, there are not many pages left you could use.

Installing a MITM is probably more common. Although I don't understand the benefit of then blocking ws, if you can read the traffic anyway?

How common is this, actually? My impression is its fairly rare, but I'd love to see some actual analytics if anyone has some.

K-12 schools definitely block HTTPS at some places, mostly because they can't inspect it.

Wow, so they can only use HTTP? That seems crazy. Would be fun to try and mess with the network though. Yeesh.

Not common anymore. Now it's usually an ssl proxy that uses a cert on all computers to mitm the connection and enforce policies.