Pearl Abyss will be giving players one more shot at a closed beta for Shadow Arena, as the game will be holding one more event in mid-April. According to the info they sent out, the beta will take place from April 17th-19th, 2020, and will be available for the North American, European, and Asian servers. Some of the new features for this phase include an improved matching system, more character balances, and normal and AI match modes. You can sign up for the closed beta here. Here's a few more details from the devs about this last closed beta for Shadow Arena.
Normal match mode is not counted toward a player's ranking. This mode is for players who want to play the game lightly without the pressure of competition. AI mode is for newer players to learn and practice while fighting against AI enemies. Experienced players can also use AI mode to practice and warm up. The special package of in-game items will be rewarded to people who sign up for the premium beta key. The package includes the highest grade sealed items, a matchlock, and Red Dragon Offering. Players who signed up for previous beta phases will receive a premium beta key and the exclusive in-game rewards.