Indeed, I hadn't realized that the parent comment I was replying to had been deleted. Furthermore, I wasn't aware that a deleted parent comment on HN would move

Indeed, I hadn't realized that the parent comment I was replying to had been deleted. Furthermore, I wasn't aware that a deleted parent comment on HN would move my own to the parent's level. I suppose you're right that PID filtering is both prone to problems and not actually available as a feature. On the other hand, Windows is able to offer per-process filtering via it's local firewall - not quite via PIDs but instead by specifying the filesystem path to the process binary. So, it must be possible to implement in some way for non-Windows.