The existing way of locking the layer does not protect its contents from unwanted change. It only provides protection before selecting from canvas area and moving contents (https://forum.affinity.serif.com/index.php?/topic/53707-lockingunlocking-layers/&do=findComment&comment=271924).   But it would also be useful to ensure protecting the layer from unwanted contents modifications. For example, as it is solved in Photoshop, with "Fully" and "Partialy" lock (https://helpx.adobe.com/photoshop/using/moving-stacking-locking-layers.html).   Related Threads