Hey @mhornung!
They OAuth changes between Boot 1.5 and 2.1 are significant (though a great change IMHO). Previously, the OAuth module was separate from Spring Security, now it has been pulled into the project proper.
One of the biggest changes is with how resource servers are configured. You can take a look at this example: https://github.com/okta/samples-java-spring/blob/master/resource-server/src/main/java/com/okta/spring/example/ResourceServerExampleApplication.java
Take a look at the ‘configure’ and ‘getUserProfile’ methods.
Let me know if I can answer any other questions!