The most important change in 1.12.4 and 2.2.1 is related to SLF4J 2.x API support. Actually the interfaces didn't change since version 1.7.x, but there was a change with discovery of logging backend. Instead of static org.slf4j.impl.StaticLoggerBinder, SLF4J 2 now relies on /META-INF/services/org.slf4j.spi.SLF4JServiceProvider which is now providing org.ops4j.pax.logging.slf4j.PaxLoggingSLF4JServiceProvider service.
From the API point of view nothing has changed except that pax-logging-api now exports `org.slf4j` package with version 2.0.6 (in addition to version 1.4, 1.5, 1.6 and 1.7). That's important for OSGi bundles compiled with maven-bundle-plugin (or bnd-maven-plugin) with default configuration which would generate this import header:
Import-Package: org.slf4j;version="[2.0,3.0)".
This would cause problems before Pax Logging 2.2.1 and 1.12.4.