RJ Studios has released True Mid/Side, Mid<>Side Channel Extractor. They say:
Traditional "Mid/Side" processing is often a misnomer for what is technically "sum and difference" processing. In this traditional approach the "Mid" output is simply the mono sum of left and right (M=L+R) while the "side" output is the mono difference of left and right (S=L-R). Any signals panned hard left or hard right - which are actually pure "side" signals - will appear in both the Mid and Side outputs. Meanwhile, the "Side" output is a mono mix containing both the left and right sides. This is not how we hear stereo and is not "true" Mid/Side.
"True Mid/Side" actually separates the phantom center (what we hear in the middle of a stereo mix) from the stereo side signals. The stereo Side signals are separated to their respective Left and Right sides. All three resultant signals (Left-Center-Right) can be exported and controlled independently. Signals panned to the center of the stereo mix (the "Phantom Center") emerge ONLY in the Middle/Center output while signals hard-panned Left/Right in the stereo mix emerge ONLY in the Side outputs (and on the same side they appear in the stereo mix).
True Mid/Side can be used to extract and render Middle/Center and Side stems from a stereo track or bus and can also be used in effects inserts and sends in your DAW during mixing and mastering. All outputs are directly usable - latency compensated and linear phase - and do not require re-encoding back to stereo.
True Mid/Side is a 64-bit plugin in VST, VST3 and AAX formats for Windows PC and VST, VST3, AAX and AU formats for Mac OS 10.11+ on Intel and Apple Silicon.
Price: $29.95. Free 14-day trial demos are available for download at RJ Studio's website.