Microsoft announces new deadline for removing the web client of 1st December 2020 in this blog and this article
Why do I have to transition?
In September 2019, Microsoft announced that the web interface (sometime called the “legacy” interface) will be removed on
1st October 1st December 2020 and that all Online customers must use the “new” Unified Interface.
There will be no exceptions so you will have to move to the unified interface.
What is the Unified Interface?
The unified interface is for model-driven apps in Power Apps (Dynamics CRM/365). It uses responsive web designs that work on a desktop, tablet or phone.
What do I have to do?
Start now by creating model-driven apps using the Unified Interface that include the components needed for your users.
Read this article from Microsoft on transitioning to the unified interface.
If you don’t do anything, Microsoft will automatically transition your environment to the unified interface as per this article
Microsoft has already started scheduling transitions from as early as January 2020.
Check the status of your instances now at the auto-transition portal https://runone.powerappsportals.com.