FeatureSpec: Nested and scope has been deprecated. Use nested feature scope itself.
Project.registerProjectListeners has been deprecated in favour of Project.registerListeners.
All Specs: All callback functions are now suspendable functions. Therefore you would need to replace override fun beforeSpec(... with override suspend fun beforeSpec(... and so on or switch to the new DSL style.
The little used afterDiscovery callback has been moved from TestListener to a new interface DiscoveryListener.
In project config, the way some settings are overridden has changed. So, instead of overriding a function, you override a val. For example, override fun assertionMode() = AssertionMode.Error is now override val assertions = AssertionMode.Error. The former methods are still present, but deprecated.
Spring and Koin extensions are now auto scanned, so you no longer need to manual add them to a project (although they won't be registered twice if you do). Simply adding those modules to your gradle or maven build is sufficient.
TestExtension now simplified to use a single callback
All duration/time based parameters are now using kotlinx.time duration.
until is now suspended