This is the current shutdown check in the migration critical section: https://github.com/mongodb/mongo/blob/77076b4121dbf89c1c7789e4c3b06e591e594d0d/src/mongo/db/s/migration_source_manager.cpp#L358-L365. However, it only successfully obviates fassert if globalInShutdownDeprecated() is true on the shard. So if the config server is shutting down and returns the error, the shard will fassert.