Is it possible to fork snap client to disable mandatory automatic updates?

There has been a certain backlash against mandatory automatic updates in snapcraft. Since the client is open source, I am wondering, is there a possibility to modify that behaviour?

No. You are using an IoT system and that always updates itself.
The basic idea is ... if you don't like IoT don't use IoT. Snaps are not mandatory and other methods of installing will also be available.

Mind that your problem is solvable by blocking updates.

sudo iptables -A OUTPUT -d api.snapcraft.io -j DROP

will block updates (create some kind of daemon to keep this in active) and ...

sudo iptables -D OUTPUT -d api.snapcraft.io -j DROP
snap refresh

can be used to manually update so I do not really understand why you would need to fork snap.