This is a sub-set of #22851 and a workaround for the time being.
Our current arrangement with Neo4j means that, as soon as reactor is on the classpath, the auto-configuration creates both a PlatformTransactionManager and a ReactiveTransactionManager. The core framework does not support such arrangement with a qualifier on @Transactional which is not something we should expect to happen out-of-the-box.
Until we figure out what to do in #22851, we're going to remove auto-configuration for the reactive transaction manager. This helps existing users test the 2.4 milestones (as the reactive support is new) and is reasonable that you have to create your own transaction manager if you want to use imperative access.
Hopefully, a future milestone of the 2.4 line will provide a better user experience for this use case.
Comment From: snicoll
I've updated the release notes to mention that limitation.