Reverse wireless charging is still a relatively new feature of very few mobile phones. In the Android 11 Developer Preview released by Google today, some developers found a similar feature called "Battery Share) ".
Users can share the Pixel's battery with other compatible devices, including smart watches, other phones, headphones, which may be supported by the Pixel 5 series.
Considering that the Pixel 4a (which may debut at the I/O conference) is a cheaper phone, the phone may not even support standard wireless charging.
Some code on the battery sharing page points to the Pixel 5. The code states that Redfin is the only phone with battery sharing enabled, and because Redfin is suspected to be the code name for the Pixel 5, it may be the first Pixel device to have reverse wireless charging.