The jakarta.enterprise.event.TransactionPhase enum that describes how the enum values relate to jakarta.transaction.Synchronization should have been in a CDI integration library from the Transactions project, but this decision was made 10+ years ago.
True, in my recollection at the time we had people thinking of CDI as EJB redone, e.g. CDI as a facade over all other specifications with all those other specifications remaining CDI unaware. Where other people were seeing CDI as the exact opposite, namely a kernel of some kind on which all other specifications could depend.
E.g. for Faces some people thought @ViewScoped should go into CDI, while others thought it should go into Faces.
At the time, different teams made different decisions, perhaps because not everyone was checking what the others were doing. Now it's clear we have positioned CDI as a kernel, and not as a facade. As such it may be time to move some dependencies away from it. E.g. @Inject Principal should belong in the Security spec, and @Inject HttpServletRequest in the Servlet spec, etc.