Jenkins core is using Guava 11.0.1, which was released on January 9, 2012. Jenkins core would like to upgrade to Guava 30.1.1, which was released on March 19, 2021. Plugins must be prepared to be compatible with both Guava 11 and Guava 30 in advance of this core transition.
Ideally, such plugins would be prepared by rewriting usages of Guava to native Java Platform functionality whenever possible. For example, instead of using ImmutableList, try Collections#unmodifiableList. Instead of using Joiner, try String#join or Collectors#joining.
If the plugin must use Guava, then limit usage to the set of methods that are present in both Guava 11 and Guava 30.