Today Autodesk announced that they are expanding the previous version access to Autodesk software. Of course, for subscription and maintenance plan customers only. This new policy change will start on 2 November 2020.
After 2 November 2020, customers can use the current version + 5 versions older. However, Autodesk still only support 3 versions back. It means now you can use Revit 2016, but there will be no more updates, hotfix, or service packs. And it’s not eligible for Autodesk support.
If it doesn’t run properly because of a new Windows update or a new graphics card driver, then nothing you can do about it. However, it’s still a good thing if you need to use an older version for a limited time until you can upgrade to the latest version.
You can see the announcement here: https://customersuccess.autodesk.com/articles/expanding-previous-version-access-to-5-versions-back-for-all-customers
I believe Autodesk made this change to encourage their customer to trade in their old license for a new subscription. Previously, trading the license means the customer lose their access to their previously own versions. For example, if you purchased Revit 2016 and keep renewing your subscription then you have access from Revit 2016 to the latest version. If you trade the license for a new subscription, then you lose access and only can use the latest version + 3 version back.
Personally, I don’t want to use an unsupported version, but sometimes it happens. Some projects are not finished yet and we can’t upgrade until all the work is done.
If you still don’t see the older version in your Autodesk account, you need to ask the older version through AVA: https://ava.autodesk.com/