Understanding the creation mechanisms for brown dwarfs has long been a stumbling block of star formation theory. Simplified arguments predicting the minimum Jeans mass fragment forming from a molecular cloud suggest a value of ~7 MJup, although more complicated considerations such as the role of magnetic fields and rotation are thought to drive this value higher. Lower formation masses are possible via secondary mechanisms. Providing direct measurements on the frequency of low-mass formation and its low-mass limit are thus key parameters needed to inform theory. An accurate measurement of the field substellar mass function can provide insight into the global efficiency of substellar formation integrated over the history of the Galactic disk. Our ability to measure the low-mass cutoff is critically dependent on measuring the space density of objects with temperatures below 500K, corresponding to late-T and Y dwarf spectral types believed to span a mass range of ~3-20 MJup. The functional form of the mass function is also most easily discerned at these same temperatures. Both of these measurements can be accomplished by establishing a volume-limited sample of nearby, cold brown dwarfs. In this talk, I will give the current status of our search for the coldest brown dwarfs within 20 pc of the Sun using both classic WISE data and the new CatWISE processing that leverages the six-year time baseline between classic WISE and the on-going NEOWISE Reactivation mission, and will update our knowledge of the functional form and low-mass terminus of the brown dwarf mass function.