It looks like the last patent on MP3 audio decoding expires next month.
You are right in that there are many other encumbered technologies that have patents expiring soon. MPEG-1 and MPEG-2 video, MP3 and AC3 audio, and several container formats are included. Notably, this is almost all of the technologies required to make a DVD.
MPEG-LA has been padding their patent portfolio by dumping in all these patents on little-used features. Until this year, they still had some good patents, such as the ones on motion estimation, which is a hard problem and is needed to make compression work. But those have now expired. What's left looks like it can be avoided as unnecessary for Internet use.
Because the interpolation operation is a full morph of a mesh, (which GPUs can do easily) you can interpolate as much as you want. Ultra slow motion is possible. You can also up the output frame rate and eliminate strobing during pans.
Kerner Optical was spun off as a separate company, then went bust. The technology was sold off, but nobody could figure out how to market it. The delamination phase turned out to be useful for 2D to 3D conversion, which was popular for a while. But Framefree as a compression system never went anywhere after Kerner tanked. Nobody is doing much with it at the moment, and it could probably be picked up cheaply. At one point, there was a browser plug-in for playback and an authoring program, but they're gone. I'm not sure who to contact; the "framefree.us" domain is dead. the "framefree.com" domain is dead. Here's its last readable version:  The remnants of the technology seem to be controlled by "Neotopy LLC" in San Francisco, which is Tom Randoph's operation.
 https://hopa.memberclicks.net/assets/documents/2007_FFV_Comp... (Open with OpenOffice Impress; it's a slide show.)