I posted on breaking a paddle shaft during float re entry practice.

I'm not finding claims, tests, or flack for paddle shaft strengths.

Of course testing paddle shafts is expensive but without merit for selling paddles ?

Is strength information available online ?

I haven't seen any 'breaking point' data on paddle shafts online. It's tough enough to get the manufacturers to be honest about paddle weights!

I think the general assumption (tested only by user experience, I think) is that most paddles are 'strong enough to do the job', but will break at some point if 'abused'.
A couple of 'rules of thumb' that I use:
One-piece paddles are stronger than ones with take-apart joints.
The cheaper and heavier the paddle type, the stronger/tougher. So an aluminum-shaft paddle is going to survive more abuse than a $500 carbon paddle, IMO.

You are not alone in breaking paddle shafts when doing self-rescue practice. I was at a (drop-in) pool session recently when somebody broke a paddle (Accent brand, 'Carbon Select' sold by MEC in Canada) right at the joint.

I think more paddles are damaged by being stepped on, than while paddling... (???)

And, as others have pointed out, the solution for self-rescue is to use 'better' technique rather than looking for an 'unbreakable' shaft support. Perhaps using a float with less flotation (foam vs inflatable) could reduce the paddle-breaking hazard (and force better technique- most foam floats will just sink if loaded with full body weight) , but IMO the foam floats are a poor second choice unless you want something you can grab while underwater to do an assisted roll (Now, that's a good 'trick' to practice!)