Oooph. Modbus is grossly inappropriate for use in IIoT architectures. It's 20 years old and has not even the slightest hint of security.

One thing we're seeing in practice is lower-powered industrial PCs running at remote sites exporting data from Modbus and other field protocols via something like MQTT on SSL.

That being said, there are thousands of Modbus-speaking (and other protocols, too) devices sitting out there on the public internet behind a cellular modem waiting to be polled or actively being polled by SCADA systems built before anybody bothered to think about security.