Can I use iPad Pro 10.5" Smart Keyboard with the iPad Air 2019?

Yes - the Smart Keyboard for the new 2019 iPad Air (10.5”) is the identical model as used with the previous 2017 iPad Pro 10.5”. Your existing Smart Keyboard will work just fine.

Compatibility of the 10.5” Smart Keyboard with both the new iPad Air and the iPad Pro 10.5” has been verified within the App Store App.

[Edit] Additional reassurance from this:

https://www.imore.com/can-you-use-apples-smart-keyboard-ipad-air-2019

Hope this helps.

Actually, the iPad Pro 10.5 Keyboard case is the right one for the new iPad Air.

Yes.

According to Apple here.

Smart Keyboard for 10.5‑inch iPad Air - US English - Apple

Actually, You could place the new iPad Air Also in The 10.5 inch iPad Pro also

If you want to buy new one

For iPad Air:https://www.apple.com/shop/product/MPTL2LL/A/smart-keyboard-for-105%E2%80%91inch-ipad-air-us-english?fnode=3e

No. The smart keyboards depend on specific connections that are only found on the pro models.

You can however use pretty much any bluetooth keyboard on your Air. (and pro too actually)