> You run a merch store. You want to share with your suppliers order data so that you can get the right number of sizes/colors/etc. Is this PII under GDPR rules? Technically, yes!

Not at all. Your shirt size is not PII. Given this information, you couldn't be identified.

> Under the new proposal, sharing this data is okay, so long as you use pseudo-anonymous identifiers (customer-1234, customer-1235).

This was okay even before. Given this information (and your shirt size), you couldn't be identified.