Java 8 to 21 + spring boot 2 to 3 brought the need to change a lot of dependencies, but often they were drop-in replacements. That was mostly Jakarta stuff. On the Spring side, a lot of things we used were deprecated, but that was not related to the Java version.

Did not take a huge amount of time to upgrade anyway. But maybe our systems weren't the most complex in the first place, a lot of our applications were pretty small.