Google Merchant Center added a new field in the "Promotion conditions" section for "first order" discount. This seems to be for shopping ads set up in Merchant Center.
This was first spotted by Chelsea Harding who posted about this on LinkedIn and wrote, "Do you offer customers a first order discount? You can now set up a promotion in Google Merchant Center for your Shopping Ads." 100% worth testing, especially during non sale times."
Here is a screenshot:
The folks on Marketing O'Clock spoke about this also, which is how I found it. Here is the embed when they started to speak about it:
Forum discussion at LinkedIn.