This comes via reliable dataminer LuigiBlood (delightful name) and reported by VGC. Apparently, the Nintendo Switch Online SNES emulator had always contained the relevant support necessary to implement the SNES Mouse, but the most recent update to NSO significantly updated the data.
The update supposedly revealed evidence of compatibility for both controller ports, the ability to do input replays of SNES Mouse devices, and code that effectively configures and changes the behavior of the SNES Mouse. It's speculated that the latter in particular will allow for the SNES Mouse to be utilized in more accessible ways than via USB.
It's all a bit vague at the moment, and truth be told, we'd be quite surprised if anything actually came to fruition from all of this. Still, it would prove a deep cut for NIntendo Switch Online and a very welcome surprise for SNES fans.
For now, we'll make do with the four new SNES games added to the online service for standard subscription members. If you've not done so already, be sure to check out our ranking of every SNES game included on the service, so you know exactly which games you need to be booting up first.