Feature summary:
Historically, the MP4 container format has been associated with non-free video and audio formats. However, as far as I've been able to determine, the MP4 container format itself isn't actually proprietary or patent-encumbered, and the container is widely supported by various free and open-source software, including web browsers.

Today, MP4 is capable of containing free video formats like AV1, VP9, and VP8, as well as Opus audio. A list of supported formats is available here: https://mp4ra.org/#/codecs

Use case(s):
Commons currently doesn't allow any MP4 files to be uploaded, even ones that only contain media in free formats. This can create confusion and an additional hurdle for uploaders who created a video file using free video and audio compression formats and chose to use the universally supported MP4 container. This can happen, for example, for users of the free video editing software OpenShot, which allows users to export AV1 video using the MP4 container.

Benefits (why should this be implemented?):
As the AV1 video format gains more traction, it's likely that a lot of software in the video ecosystem will choose to use MP4 rather than WebM as the container for it. Supporting MP4 for free formats would simplify uploading video to Commons, a process which is already quite complex for the average user.