Just because it's a "smart" service doesn't mean it has to connect to the Internet or a server or the manufacturer. If it does neither, it can't be turned off by them.

All my devices run local-only protocols. Nothing leaves my house. The devices that would be proprietary were reflashed to tasmota (fully open source, local only). Others are either Zigbee or Shelly. While Shelly has a cloud connection, it's fully optional and disabled by default (including automatic updates). The hardware is also supported by tasmota, and reflashing is always just 5 minutes of effort away.

There is absolutely nothing that any manufacturer has to do to keep my stuff working. I have to do a little something (keep my tiny server on, basically). But more importantly there is nothing any manufacturer can do to stop my stuff from working.