The snap was recently deprecated, which is why it’s no longer available in the store. In hindsight we should have posted a notice on this here ahead of time, and I’ll take the blame for that oversight.
The source code is available here:
The alternatives to building your own version of the snap is to use stage-packages to include udisks2 snap into your own snap, and then add your own logic to handle the automounting logic. udisks2 itself just monitors block devices and provides a DBus interface for other applications to introspect/monitor them. The snap also included some code which originated from the Ubuntu Touch project which actually performed the automatic mounting of USB drives when detected via the udisks2 DBus interface.