The Forms Recognizer 3.1 is expected to be available as a container later this year. According to the information from Microsoft Q&A and other sources, while the exact release date has not been provided, the product team has confirmed that the new containers supporting v3.1 APIs will be released soon within this year.
For the most up-to-date information, it's recommended to keep an eye on the "What's New" page of Azure AI Document Intelligence and other official documentation from Microsoft.
More links :
https://learn.microsoft.com/en-us/answers/questions/1380951/custom-neural-container-availability