SSD with PCIe 3.0 compatible with MBA which uses PCIe 2.0? Have 13", 2017 Macbook Air currently with Mojave OS. Is an SSD with "Ultra-Fast PCIe NVMe Gen3x4 Interface" (for example recommended SSD from Tom's Hardware) likely to be compatible with a Macbook Air if I use a physical adapter designed to handle the Apple proprietary blade configuration to the M.2 blade standard "conversion" (e.g. Sintech NGFF M.2 nVME SSD Adapter Card? I'm wondering whether the PCIe 3.0 protocol on the board is going to be backward compatible with the PCIe 2.0 that the MBA uses.

I found a similar question, but I thought the answer was specific to that questioner's situation, since the suggestion to use a USB access to format an M.2/PCIe board would require an extra enclosure. I'm pretty sure the more general answer would be to construct  a bootable USB stick and format the board once it's in the computer (and that's not really my question, since I found lots of instructions on how to do that.)

Note: thought PCIe would be a reasonable tag but my low rep on this forum prevent creating a tag.)
Yes, it is likely to be compatible - however there are (ofcourse) no guarantees.

According to the manufacturer of the adapter, they have tested that specific SSD with the adapter - so that sounds reasonable.

I would consider using this specific adapter from the same company:

https://www.amazon.ca/dp/B07FYY3H5F/ref=dp_cerb_3

It is listed as supporting your SSD. In addition ifixit.com (well known and respected site) lists this particular adapter as tested with your model of MacBook Air - though with a Samsung Evo 960 SSD.

https://www.ifixit.com/Guide/MacBook+Air+13-Inch+Early+2017+SSD+-NVMe+Replacement/121102