I guess @eosfan took a chance, as sometimes one has to do, and got lucky. Without detailed knowledge of the final software revisions for guacomole one cannot be absolutely sure that the vendor supplied with e-t-xxx builds will completely overwrite pre-Android 12 versions, for all variants.
- Manoj has suggested elsewhere that this sort of experience could be recorded in #ewiki:documentation-suggestions guacomole even so @eosfan’s experience might only work with the variant he is using.
@mauronofrio at XDA in this 2019 thread https://xdaforums.com/t/rom-stock-fastboot-op7p-stock-fastboot-roms-for-oneplus-7-pro-7-pro-5g.3931424/, provides links to Fastboot ROMs which he has curated.
I have extracted all images from the stock zip and i have made a new zip with the Fastboot ROM with a flash-all.bat included.
However in general these are older or original stock versions and one would need to update them, possibly with the third party app Oxygen Updater.
He further comments
OnePlus will no longer release ROMs flashable via stock recovery because is no more needed. The updates will be done on the slot not used …
This link is primarily about unbricking [Guide] How to use MSM tool. Derived from that link is this index from the OnePlus Community Server. Many links within it are to the work of @Some_Random_Username at XDA.
As always it is informative to check the Lineage Wiki
- https://wiki.lineageos.org/devices/guacamole/install/ includes please ensure that the device is currently using Android 12 firmware.
- https://wiki.lineageos.org/devices/guacamole/fw_update/ suggests the latest Android 12 full stock ROM update from Oxygen Updater.