We have implemented a new linter called 'forbid-tasks-with-tag-on-variants'. This linter's main function is to forbid tasks with specific tags from being used in buildvariants with certain tags.
 
Currently, the 'streams_kafka' and 'streams_kafka_gwproxy' have an 'experimental' tag, but it has been added to the "Amazon Linux 2 enterprise build with streams" build variant. This build variant is considered non-experimental because it includes tasks of type '.release_critical', '.development_critical', '.default' and etc. 
 
To resolve this issue, I have a couple of ways in mind like:
1: Update the tag for those two tasks
2. Remove those two tasks from this build variant
Please choose the option that works best for your team and fits the purpose of this task.

Here is more info about Task Tags
 
I have currently added those tasks to the 'ignored_tasks' list in the evergreen-linter rule 'forbid-tasks-with-tag-on-variants' so it will not trigger any problem for now. However, please update them whenever your team can

Here is my PR with more info.