I am also receiving repeated messages about this issue. I followed the advice mentioned below and viewed the robots.txt extension on my store and competitor stores and found that my store and some others on shopify have a long list of items that are blocked. Whereas some other stores have very few lines of code in robots.txt.
I raised this with the shopify support team and they say that the long list is recommended by them. I’m still not 100% convinced as the CSR didn’t seem to have any knowledge of this issue.
Any more words of advice would be welcome.