After latest update when in region mode tone curves you can't add any points. In the RGB mode it works fine
When in the 'region' mode, you have never been able to add any points to the curve. Points can only be added to the curve by clicking the small Point Curve button in the lower right of the Tone Curve panel. If the button is depressed, points can be added. If it is not depressed, points cannot be added.
Jez, as long as you're in Point Curve mode (no sliders, just Point Curve pop-up), then you should be able to click on the curve to add points even in Linear or Custom in the pop-up. I've just confirmed it's working on Mac in LR Classic 7.3. If that's not working for you, can you confirm your exact version (Help menu > System Info) and OS please?