Android's source code is a maze of branches and tags, unfortunately. The change can have been made in the generic cross-device kernel tree, or it could've been a separate module, or it could've been a binary.
One thing you could consider is checking out the kernel for a project like LineageOS: https://github.com/LineageOS/android_device_xiaomi_sm6150-co... They usually pull+filter changes from upstream. For instance, these changes may be of interest: https://review.lineageos.org/c/LineageOS/android_device_xiao... https://review.lineageos.org/c/LineageOS/android_device_xiao... These kernels are used for a variety of devices with similar hardware, so not all changes have anything to do with the Pixel 4a, but it could prove useful in your search.
