I was told to supply a letter disclaiming the need for disability support otherwise I wouldn't be able to carry out data collection. Is this normal practice?

Of course not.

You need to go to the disability services department within your university and ask for the support you need and then your supervisor will have to make 'reasonable adjustments' to assist you. If you're not sure how to do this, go through the postgraduate administrator in your department.

My university is very supportive: I have received help from the disability service and transcription service. It is my PhD supervisor who wanted this letter otherwise I couldn't do data collection for my PhD

Hmm, seems very odd. I would discuss this further with your supervisor and find out why they are saying this. Doesn't seem right to me. If they are not forthcoming then you could go and speak to admin and see what they advise.