Promotion at Google, as in many places, is tough. Status is allocated partially on level, so it sucks to not see that growth.
Sometimes lack of promotion can be not having the right opportunities.
It's fair to leave a company for whatever reason.
For any other L4->L5s, or anyone wanting to become a senior engineer, it's worth self reflecting on whether there's improvement that can be made from failed promotion attempts.
> people all across the org knew me and said I was indispensable to the company and were surprised that I wasn't already at an L5/6 level.
No one in a large org is indispensable, but many are very valuable. Many L4s are very valuable, but at doing L4 work. It's not a value judgement.
L4->L5 is a step of responsibility: can you be trusted to handle a multi quarter project, without much supervision.
> I helped launch/lead features on YouTube, I led teams, I designed and implemented systems that were still in use to that day by many people
The details aren't clear here, but sometimes an engineer can be leading projects, and need supervision: poor delivery, poor communication, poor outcomes.
"Too little impact" in this context can mean "you needed too much supervision" or "too little impact per $TIME_PERIOD" meaning you can have delivered great technical solutions, but not at the rate or level of independence needed to meet the mark.
Again, not meeting this mark isn't a value statement. It's a different type of work, but it happens to be incentivized with more $$$.