So according to this post a rider with a lower level than a course included in an event can ride the course in that event anyway.
According to Zwift support, that’s not possible anymore:
I’m sorry, but you’re correct in that you will not be able to participate in the event until you reach level 12. At this time, the Alpe Du Zwift is level restricted. If you aren’t at the designated level you will not be able to ride the course, regardless of if you’re in an event or not.
At the very least, it shouldn’t be too difficult to compare two numbers when someone sign up for an even and bring up a warning stating that they need to be a certain level before the event start into order for them to be able to ride the entire event.
As it stands, if I hadn’t asked support, I would have been joining this even tomorrow and would have been stopped 3/4 of the way because I can’t follow everyone else (I’m currently level 11).
This seems like less than desirable UX.