As of 2015, setting up a formal migration path to a beta site was not something SE was willing to do. (Questions can still be migrated manually by moderators from anywhere to anywhere, though.) But the meaning and significance of the "beta" label on a site, as well as the criteria for a site to be decommissioned or graduated, has changed a lot over the years.
Is SE now (2020) willing to set up formal migration paths to beta sites, if requested?
(Context: I'm a moderator of a beta site [Literature] which receives a lot of migrations [38 total in the 3.5 years of its existence, 7 in the last 90 days] from English Language & Usage, mostly due to active Literature users patrolling ELU and flagging things for migration. I've recently started a meta discussion about whether Literature wants a formal migration path from ELU, but another user raised the question of whether this is even possible while Literature is in beta.)