Updated to GCam version: 18.104.22.1684820638.44 on my Pixel 4a and now it’s possible to lock focus of the camera.
You would not know about it unless you activated it by accident of course because “Bug fixes and performance improvements” in the update description.
To lock focus tap and hold inside the view finder until a [yellow square outline with a lock appears](https://i.imgur.com/3rvZI2z.png). It works in all the modes I’ve tried (normal, portrait, night mode, video).
Took them a while but I’m glad it’s finally available. Unfortunately we do have to sacrifice the quick access to Google Lens as focus lock gesture replaces it.
They also added another feature. If you tap and hold on the thumbnail of the last photo you took, [it’ll offer you an option to save the next photo to a locked folder](https://imgur.com/eBXpr6x.png).
Hopefully this is a small step towards adding more features and eventually a dedicated pro mode where users will have more granular control over focus, exposure, temperature, ISO, and shutter speed with better controls than [current imprecise levers](https://i.imgur.com/JjUS0nS.png).