Why is Guava's EventBus marked unstable in IntelliJ 2018.2?

Because the EventBus class is annotated as @Beta.

You'd have to ask the Guava project maintainers why a class that exists for 16 versions of Guava is still Beta, but it still is.

Besides @JB Nizet's answer that perfectly explains why IntelliJ flags this error, you may also want to suppress it globally but only for the Google Beta annotations.

Just go to Settings -> Editor -> Inspections -> JVM languages as per the picture bellow and, if you feel like it, delete the com.google.common.anotations.Beta.

You can also ask IntelliJ to suppress the warning for the definition of your @Beta annotated class as follows:

@SuppressWarnings("UnstableApiUsage")
static RateLimiter API_RATE_LIMITER = RateLimiter.create(8);

This properly deals with the warning in a particular case when you're okay with using the class anyway (and don't run into issues).