It's already open source, because GPL requires it. It's unlikely to be accepted as an upstream contribution given that Oracle has their own Vector type that is only available in their MySQL cloud service. Don't expect it to show up in a standard Oracle MySQL community release.
I have the same question. I have a small DB on Planetscale and happily pay 39 for it. IMO, Vector DB's are the next frontier. I can understand the tough decision for Planetscale, given that the cloud services can just offer it as a service and corner all value of an open source project.