You're basically evaluated according to L5 requirements. Given that you weren't promoted at that point, it's likely you're not performing at those requirements, and you'll be slowly managed out. More often as people get close to the red zone, they just swap companies preemptively.
AFAIK, Google has a very similar policy. At FB, E5 is a "terminal level", so once you hit that level, there's no push or expectation to reach beyond that. I've been in the E4 red zone at one point, and spent about a year getting evaluated at E5 levels before getting the promo. You're not automatically out if you don't reach it in time, but you have to be showing progress, and it does get more urgent the longer you spend in that zone.
I used to manage a team at Google. L3 basically meant you had to be told what to do. L4 meant that while you may not regularly give a lot of strategic direction to the team, you could be relied upon to just pick up the ball and carry it forward toward shared objectives. Your code was good enough to not require a bunch of time- and energy-wasting back-and-forth with reviewers. What you delivered stood on its own and was more an asset than a liability to the team and the product.
If, after a few years, you still need to be told what to do in order to be productive day-to-day, you’re simply not developing, and unless you figure out what’s going on you’re probably going to struggle to build a successful career.
For my own team, the bar I set for giving higher assessment ratings was how well you could make and keep commitments to the team. If, after working in the code base with the group of people for a year or so, you couldn’t independently come up with useful things to commit to doing and couldn’t deliver in the timeframes you said you could, I would start asking you whether you are really happy and motivated doing what you’re doing with your career. For the few times I had to do that, the member of my team ended up admitting to themselves that they were making a mistake trying to do this type of work, and they moved on to something else where they were more happy and successful.
What happens if you don't?