Is Spring Boot just for Microservices or can I use Spring Boot for Monolithic architecture?

Spring Boot in itself has nothing to do with microservices. It's a Spring module which simply makes the configuration of your app easier. As such, it absolutely can be used in a monolithic app.

From the official docs:

Spring Boot makes it easy to create stand-alone, production-grade Spring based Applications that you can "just run".

We take an opinionated view of the Spring platform and third-party libraries so you can get started with minimum fuss. Most Spring Boot applications need very little Spring configuration.

Features

• Create stand-alone Spring applications

• Embed Tomcat, Jetty or Undertow directly (no need to deploy WAR files)

• Provide opinionated 'starter' dependencies to simplify your build configuration

• Automatically configure Spring and 3rd party libraries whenever possible

• Provide production-ready features such as metrics, health checks and externalized configuration

• Absolutely no code generation and no requirement for XML configuration