It was removed from Spring Framework, and thus Spring Boot.
https://github.com/spring-projects/spring-boot/issues/10008
Velocity support has been removed as of Spring Framework 5 and therefore our support as well (even deprecated in earlier version). As far as I can see there is no spring support in Velocity 2 so you may want to ask that to them first.
The relevant Spring Framework information,
https://github.com/spring-projects/spring-framework/wiki/What%27s-New-in-Spring-Framework-5.x#removed-packages-classes-and-methods
Dropped support: Portlet, Velocity, JasperReports, XMLBeans, JDO, Guava.
Recommendation: Stay on Spring Framework 4.3.x for those if needed.
Why the decision was made I cannot answer, there is always various reasons for dropping support for other frameworks but perhaps it wasn't worth the maintenance burden on the framework.
Update,
There is now a Spring support module.
https://downloads.apache.org/velocity/engine/2.3/release-notes.html
New spring-velocity-support module for Velocity Engine integration in Spring Framework.
For complete details please see.
https://issues.apache.org/jira/browse/VELOCITY-933