I gave this a bit of thought
Can anyone point me to a resource for evidence? (That helmet normalisation deters [would be] cyclists)
I don't think this is possible to determine. The process of normalisation has happened gradually alongside many other changes, so you'd be hard pressed to attribute any deterrent effect from any other causes. Unlike, say, compulsory helmet legislation - where you can do before/after analysis.
Along with habituated helmet wearing comes the message, either spoken implied, from mum and dad that such activities are dangerous. That itself may (I say may, because this is pure supposition) be equally effective at turning youths away from a dangerous activity like cycling.
I think you have hit the nail on the head. Helmet normalisation is part and parcel of an overall increase in the perception that cycling is dangerous. I don't think you can disentangle them. Someone deterred from taking up cycling because they feel a helmet to be necessary, and don't want to wear one is very likely to be someone who is also deterred from cycling because they perceive it to be dangerous.
Where this perception comes from, and whether it is justified, goes beyond the bounds of the OP.
So my thesis is that helmet normalisation is
not a cause of would be cyclists being deterred. The normalisation of helmets and the deterrence both result from the same root cause, which is perception of cycling as a dangerous activity.
Now to go and gather some evidence for my thesis. Oh bugger.
Full disclosure: I'm a habitual helmet wearer. My reasons are my own and none of your business.