Spring 5 introduced Reactive Spring Data (RSD) and there are a few different clients which are currently supported (MongoDB, Apache Cassandra and Redis).
You can read more about RSD here https://spring.io/blog/2016/11/28/going-reactive-with-spring-data.
As much as Spring is a standard, I think that part of it's popularity could be it's use of standards such as JDBC.
It's totally possible that RSD, still being in it's early days, that other frameworks and libraries will being using it.
I think that you may be able to use RSD even if there is no supported client for the database in question (but I could be wrong).
Have said all of that, I strongly suspect that currently many of the Spring projects out there will not move to RSD for some time.
Is now the time to learn RSD and start using it?
“The strongest of all warriors are these two — Time and Patience.” ― Leo Tolstoy, War and Peace
This the 'best' time to get into reactive Spring. Reactive support for JDBC will be there once reactive JDBC drivers (which are being developed) are available. Btw, these days many applications don't even use typical JDBC databases.