The AOSP Calendar is in a repository tool (Google Git) that use Google with a lot of project.
Compile it by yourself
You could download it:
git clone https://android.googlesource.com/platform/packages/apps/Calendar
and you could select the branch/tag that you want.
git fetch origin nameBranch/nameTag && git reset --hard FETCH_HEAD && git checkout nameBranch/nameTag
Of this way, you obtain the code of the Calendar AOSP but as you said, in this repository is not the package of the application, so, you should compile it. As it is part of the Android system is neccesary to compile it with the rest of AOSP, because, it has some dependences.
LOCAL_STATIC_JAVA_LIBRARIES := \
android-common \
libchips \
colorpicker \
android-opt-timezonepicker \
androidx.legacy_legacy-support-v4 \
calendar-common
Once you had it compiled, you don't have problems to install it, because althought it is a part of AOSP it is not a system application.
But, this is not a quickly way to obatin the APK file.
Obtain it of system.img
First, you need to obtain a AOSP or Lineage ROM system image before the 2019 (because, the commit that you don't want is in 2019). For this way, you need to use this tool. Once downloaded and installed:
mkdir system_lineage
simg2img system_lineage.img system_lineage.raw
sudo mount -t ext4 -o loop system_lineage.raw system_lineage
cd system_lineage
And you need to found it in this system directory (I think that it was in system/app/Calendar or something similar)
Try to obtain it in some mirror
It is possible that some mirror have this apk in the server. You could try to obtain it in apkmirror for example.