I have confirmed that it is possible with URP of Unity2020.3.23 and Unity2021.2.7.
(1) Set the Render Texture of the Camera to a format with an alpha plane such as R16G16B16A16_FLOAT.
(2) Turn off Post Processing of the Camera.
(3) Alpha plane does not become 1.0.
(4) Implement necessary post processing with Render Features.