The other projects who support some of these older devices have numerous issues as noted here: https://divestos.org/index.php?page=patch_levels#osSecurity
Edit: also of note: DivestOS currently provides monthly updates spanning seven versions of Android, I don't know of any other project doing that specifically.
Don't get me wrong, it's terrific that security patches are backported to such ancient versions of Android by those working on DivestOS and it's a great option for devices that aren't supported by GrapheneOS, LineageOS, et al.
https://gitlab.com/divested-mobile/firmware-empty/-/blob/mas...
This is indeed an issue and is documented on multiple places of the website.
Patching everything else is the best harm-reduction for this.
Now if you're buying a device planning to run it, that's fine, but it really does limit its usefulness.
I recently got an unofficial build of LineageOS running on a Nexus 4 (mako) device and I was positively surprised with the speed it can run modern software. But this is an unofficial build that is also broken on some essential points, such as WiFi.
For these old devices, Graphene is not an option and if there are others targeting the same devices as DivestOS (which I will surely be checking out soon) I have yet to see them.