PlatformTransactionManagerCustomizer was introduced in 1.5 before Framework introduced its TransactionManager and ReactiveTransactionManager interfaces. This means that it's only possible to customize a PlatformTransactionManager and not a ReactiveTransactionManager. With the introduction of ConfigurableTransactionManager that is implemented on both sides in Framework 6.1, we should broaden our support for customization to support any type of TransactionManager.
This change will allow us to implement #36770 using a customiser-based approach.