Does Syncthing send ignored files?

Sync thing scans some (all? unpredictable?) ignored files. As of 2022-07-27 this is just part of the personality of the program.

But does it potentially send ignored files over the network?

What happens when devices have different .stignore lists?

It does not. You can test this yourself by creating a new folder while also setting its ignore patterns to * at the same time. Nothing will be scanned, and the database will remain unchanged too.

What’s ignored isn’t sent to other devices.

Ignore patterns are local by definition, so you can have them completely different on each device. The global state is going to be a sum of everything what’s not ignored.