The Kingdom Hearts: Missing Link closed beta test will appear sometime in Fall 2022. According to an interview with Famitsu, the mobile title will feature a closed beta period. However, more information on this will appear in the future. That said, the interview also disclosed a key staff member that will be working on the narrative of Kingdom Hearts: Missing Link. [Thanks, ryokutya2089!]
Ruiko Fukazawa, who has writing credits on Final Fantasy XIV: Heavensward, will be working on the main scenario for the mobile title. However, the bit of the interview that is available does not disclose further information about the staff or the contents of the narrative.
What we do know about Kingdom Hearts: Missing Link is that players will assume the role of the protagonist. There will be character customization, similar to Kingdom Hearts Union X. Kingdom Hearts series Director Tetsuya Nomura also spoke briefly on the potential of Disney worlds appearing in Kingdom Hearts: Missing Link. He stated that the team is still in the process of properly implementing these worlds. Originally, areas were covered almost completely in Disney motifs, but Nomura felt as though this was a bit overkill. As a result, some areas may feel more inspired by Disney properties.
Kingdom Hearts: Missing Link is in development for Android and iOS devices, with the closed beta test scheduled for Fall 2022.