I also contacted Apple's Customer Support and the person I talked to says they couldn't find any documentation to confirm the material used for the display on the Series 6. None of the other agents around them could confirm that either. They said most likely it would remain the same as the previous generations (sapphire for steel and titanium, Ion-X for aluminum); however, they said that since they could not confirm this information, to just wait till the watch is physically available where someone will be able to confirm the material by looking at the backside. I couldn't find any promo material that showed the backside of the stainless steel model. The AR model on Apple's website is the aluminum model. Even Apple's technical documentation has not been updated to show the materials. Here is a link to the support page that shows material compositions of their watches for people with skin allergies: https://support.apple.com/en-us/HT204665
For now, it's a waiting game (albeit a brief one).