We plan to start rendering fallback markup in IE9 and IE10 for all Twitter for Websites widgets in the very near future. We will continue to fully support IE11 and the successor Edge browser.
We came to this decision for a variety of reasons. Our data studies clearly show us that usage of Twitter embeds in these browsers has dropped significantly in the past few years. Meanwhile, the lack of full Flexbox support has made it difficult to iterate on Tweet and timelines embeds and keep them up to date with modern trends in web development. In addition, Microsoft announced end of life for both IE9 and IE10 in 2016.
Whenever possible, we always do our best to support platforms irrespective of their browser share, so long as they support modern web standards. When this is not possible, we always do our best to gracefully fallback to the embed code. With the embed code fallback, your users will able to see the fully rendered content by clicking through to Twitter.com.
If you have any questions or concerns about this change please let us know via the Twitter for Websites category on the developer forums.