No Spring Boot isn't just for REST APIs.
Spring Boot is "just" a mechanism for autoconfiguring a Spring Framework based application.
Therefore you can use and it does get used for all kinds of stuff.
- REST APIs for webservices
- Full web application using Spring MVC
- SOAP services (or are they called SOAP dispensers?)
- Reactive web applications
- Command line tools
- Batch jobs
- Swing / JavaFX applications
...
Of course there are many more people writing web applications than Swing applications with or without Spring.
The kind of web application you describe and which I put under "Full web application using Spring MVC" is a very well established model and when done right way better aligned with the principles of REST than the average so called REST service. My very personal guess is: They will still be around when nobody remembers what Angular is.
For your additional question:
Your question sounds a little like the relation between JPA and Spring Data JPA might not be completely clear.
(see Spring Data JDBC / Spring Data JPA vs Hibernate)
Both are certainly used in real world projects. By definition more projects use JPA than Spring Data JPA since the first is a superset of the later.