История изменений

Что на практике эквивалентно всем процессам, запущенным от одного пользователя.

Вряд ли.

In Linux systems where POSIX capabilities are used, the ability to ptrace is further limited by the CAP_SYS_PTRACE capability …

https://en.wikipedia.org/wiki/Ptrace#Uses

Linux also gives processes the ability to prevent other processes from attaching to them. Processes can call the prctl syscall and clear their PR_SET_DUMPABLE flag; in later kernels this prevents non-root processes from ptracing the calling process;

https://en.wikipedia.org/wiki/Ptrace#Linux

CAP_SYS_PTRACE
...
transfer data to or from the memory of arbitrary processes using process_vm_readv(2) and process_vm_writev(2);

https://www.man7.org/linux/man-pages/man7/capabilities.7.html

Наличие ptrace не отметяет наличия виртуального адресного пространства для процессов.

Что на практике эквивалентно всем процессам, запущенным от одного пользователя.

Вряд ли.

In Linux systems where POSIX capabilities are used, the ability to ptrace is further limited by the CAP_SYS_PTRACE capability …

https://en.wikipedia.org/wiki/Ptrace

CAP_SYS_PTRACE
...
transfer data to or from the memory of arbitrary processes using process_vm_readv(2) and process_vm_writev(2);

https://www.man7.org/linux/man-pages/man7/capabilities.7.html

Наличие ptrace не отметяет наличия виртуального адресного пространства для процессов.