Which keyword do I need to use in expire_in to keep an artifact forever?
Set it to 0.
You sure it works? In any case, there seems to be a feature request for this being worked on.
For GitLab.com the artifacts never expire, for your own instance there’s a setting in the admin area https://docs.gitlab.com/ee/user/admin_area/settings/continuous_integration.html#default-artifacts-expiration.
I’m assuming it works the same for .gitlab-ci.yml 
Can you link to the issue?
I think @pmatos meant this issue here.
If you try:
expire_in: 0
or
expire_in: "0"
you get error “expire in should be a duration”.
There’s also issue to allow keeping the latest artifact forever, which would be great in some use cases.