Therein lies Google's problem, as once anything is usable in public it does get used/abused. There are certain VM behaviours in compatibility mode which are like this, where outright bugs have to be preserved in later versions for the benefit of apps which aren't going to receive timely updates.
1. https://android.googlesource.com/platform/system/core/+/mast... line 218
The above makes sense if you want to assume that you need /proc at least at some permission level. If you don't (which I think you don't), then it should simply be unmounted in the sandbox.
IPC is the standard way to get out of the sandbox, and should be used also for whatever features the APIs need /proc for.
Also, /proc nor ptrace is "useless". Both are actually fundamental to several aspects of the underlying system that zygote runs on.
It sounds like the actual Intent contents remain secure, however, the creation of an Intent while a particular app is in the foreground and using a particular amount of memory might be the kind of trigger they'd be looking for.
An OS level patch to get around this could involve preallocating shared memory for each application so that differences in it during the app lifecycle do not actually occur. Modern systems have enough RAM, and there are low enough limits on what is flying around, that this shouldn't be a problem.