Question: And how are we going to use Velocity with Spring after Spring 5.0?

Answer: 
It looks like Jürgen Höller wants to get rid of Velocity alltogether, because it "dates back to 2010".
Support for it was deprecated in Spring 4.3, and will probably be removed in Spring 5.

<https://jira.spring.io/browse/SPR-13795>
In the accepted answer from @Olivier Croisier he states that the reason for depracating velocity was that it "dates back to 2010". While this doesn't seem a particularly valid reason to get rid of something (The C language dates back to 1972) that was actually Jurgen Hoeller's reason given in the issue description that Olivier links to!

In further comments added to that issue, that appear after @Olivier Croisier's answer was given, there is further elaboration. It seems Spring wishes to limit the 3rd-party adaptors that they provide to only two implementations and suggest that the other 3rd-parties develop their adaptors themselves.

The impression I get is that velocity was neither popular or modern enough to be selected as one of the two.