Locking layers in Affinity currently "locks transformations" but should make the contents of a layer "read-only".
While a transformation lock might make sense in Affinity Designer, it is definitely not sufficient in Affinity Photo. The current behavior is confusing for many users:
In Affinity Photo, this issue is particularly frustrating because layers tend to take up the whole canvas when editing photos.
As an example, I often alt-click to isolate a layer or mask. When I'm done with that particular mask, I click on the background to see the whole image. If I'm not attentive, I can draw on the background layer and only realize when it's too late. Yes, I should pay more attention. Yes, it's my fault that I'm easily distracted. But in other software, I can rely on locks to stop me from making such mistakes. Affinity promises me a lock but gladly destroys the original image.
I'm not alone in being surprised by this behavior as the link above shows. Users are being confused for more than 3 years now. The layer lock in Affinity fails the principle of least suprise.
I'm sorry if this post sounds like a rant. I find the Affinity Suite an otherwise fantastic piece of software. It's just frustrating that such a glaring oversight is being accepted as "by design".