The traditional guidance for setting default migration paths focuses on the level of migration occurring.
There's no "right" level for establishing a migration path, but tens or hundreds of migrations with a single digit rejection rate seems safe. Single digit migrations in the last 90 days and/or rejection rates that can order a drink without ID are signs that the path will be a waste of time to establish.
While others have pointed out that sites in beta are almost never added as default targets, moderators can still migrate them. And a SEDE query for questions successfully migrated away doesn't make much of a case for LL:
User Experience 10
Right now, LL is in a 4-way tie with Academia, Cognitive Science, and, umm, Graphic Design, with the last successful migration all the way back at 2017-09-05 08:42:41.
If you do come across questions that would be a stronger fit for LL, just flag them for moderator review as others have suggested, and that will improve the case for the community migration path if and when LL leaves beta.