Microsoft announced in September 2019 that legacy authentication would be disabled on October 13, 2020, however due to the corona virus Microsoft has pushed the date back to the second half of 2021. The exact date of when the change will be pushed is not known and will be communicated at a later time. However newly created tenants or tenants that have no recorded use will have legacy authentication disabled starting October 2020.
Disabling legacy authentication is still a best practice and should be disabled for the entire tenant. Don’t wait for 2021, use Conditional Access to block legacy authentication and see how your services and users are affected. Conditional Access requires an Azure Premium 1 license which is part of the Enterprise Mobile + Security (EMS), Microsoft 365 or a standalone license. Read the documentation on how to disable legacy authentication using Conditional Access.