Thanks Emmanuel.
I have reached out to Shopify support but after 3 emails, and 2 chat conversations, all they have been able to tell me is that the robots.txt file cannot be edited (which I already knew).
I’ve just sent them an email explaining the issue of product pages not being crawlable, and asked for the issue to be investigated.
Is there any way you know of to get issues escalated to Tier 2 support at Shopify?