To save screenshots in lossless WebP, the compression quality should be set to 100%. (Reference: Bug 420449 Comment 4)
However, there is no way to set it to 100%.
STEPS TO REPRODUCE
1. Open Spectacle and click "Configure..." -> "Save"
2. Drag the Compression Quality slider
3. In the textbox of Compression Quality, click the arrow and try making it 100%
1. When dragging the slider, the maximum value is 99%.
2. When using the textbox to set the value to 100%, the Apply button will not be activated. When you click OK, close Spectacle, start Spectacle again and go to the settings, you will find that the maximum value will return to 99%.
If I edit compressionQuality in spectaclerc to 100, then the webp output will be lossless, but as soon as I go to Spectacle settings again, the value returns to 99%, and Apply button is activated (even though I have not made any changes in the GUI).
A possibly relevant merge request was started @ https://invent.kde.org/graphics/spectacle/-/merge_requests/53
Fixed by Natsumi Higa with https://invent.kde.org/graphics/spectacle/commit/afdb19d5358c860e5c6617dc9270167aeed551a9 in Spectacle 20.12.3!