It's still possible to lease dedicated lines

Not in the uk it isn't.

More cost effective to unilaterally bung eveyone on an ip based system, which is unacceptable for any secure/time critical application.

AFAIK stux moved via usb keys - tyipically SCADA isnt connected by TCP/IP - usually a modern variant of RS 232. So code gets developed & tested in a sim, and shipped to remote sites in some dudes pocket on a flashdrive.