Reverse wireless charging is still a relatively new feature that not many phones have, but it looks like it'll be coming with Android 11 for some future phones in the way of a feature called 'Battery Share.'
The description in an image that was discovered states that users can share the power from their Pixel device with other compatible devices. Which could include smartwatches, other phones, earbuds and more.
This is likely a feature that will become available with the Pixel 5. The Pixel 4 series is the latest device from Google (though the Pixel 4a is expected to make a debut at Google I/O this year). And it doesn't support reverse wireless charging.
This suggests Battery Share is for a Pixel device that has yet to release. Considering the Pixel 4a series (if unveiled at I/O) is meant to be a more budget-friendly phone, it's unlikely it would have even standard wireless charging.
Google only enables battery share in Android 11 for 'Redfin'
Redfin is one of the codenames for an upcoming Pixel phone. It's suspected to be the Pixel 5, as Sunfish is suspected to be the Pixel 4a.
As mentioned above, the 'a' series of Google's Pixel phones are less expensive versions of its flagship offering. So wireless charging isn't something that would likely be included. Let alone reverse wireless charging. Which is what battery share clearly is.
According to 9To5Google, some code found within the battery share page points to the Pixel 5. The code states that Redfin is the only phone that battery share will be enabled for. And since Redfin is suspected to be the Pixel 5, then it may be the first Pixel device to have reverse wireless charging.
Battery share will eventually be available on non-Pixel devices
While Battery Share may be an Android 11 feature that launches with the Pixel 5, it won't be exclusive forever.
Eventually, Android 11 will be made available for non-Pixel devices. And there will no doubt be some that come with reverse wireless charging. So it stands to reason that eventually the battery share feature will work with those phones.
That's also if there isn't a proprietary version of it that's already implemented. Samsung for example already offers it through its own version of Android. And other phones can of course do the same.
Since this feature also requires specific hardware to work, it's not something that will come to older Pixel devices that have regular wireless charging.