WifiDiagnostics files filling up drive even with logging disabled
(Answering my own question, since others seem to be having this issue as well.)

The "hyper+w" (command+option+control+shift+w) key combo is triggering the the wifi logging. Note that "hyper+>" also seems to trigger this. I haven't been able to figure out why it's happening, but that's the culprit.

I've used Karabiner Elements to disable those keyboard shortcuts entirely, to prevent accidentally triggering this wifi logging. Here's the relevant section from the karabiner.json file to make those key combos trigger an escape instead of the wifi logging. To add these rules, go to the "Misc" tab, and do the export/import "Open config folder", then edit the karabiner.json to include the following two rules.

"rules": [
    {
        "description": "Disabling command+control+option+shift+w. This triggers wifi logging.",
        "manipulators": [
            {
                "from": {
                    "key_code": "w",
                    "modifiers": {
                        "mandatory": [
                            "command",
                            "control",
                            "option",
                            "shift"
                        ]
                    }
                },
                "to": [
                    {
                        "key_code": "escape"
                    }
                ],
                "type": "basic"
            }
        ]
    },
    {
        "description": "Disabling command+control+option+shift+&gt;. This triggers wifi logging also.",
        "manipulators": [
            {
                "from": {
                    "key_code": "&gt;",
                    "modifiers": {
                        "mandatory": [
                            "command",
                            "control",
                            "option",
                            "shift"
                        ]
                    }
                },
                "to": [
                    {
                        "key_code": "escape"
                    }
                ],
                "type": "basic"
            }
        ]
    },
    {
        "description": "Change caps_lock key to command+control+option+shift. (Post escape key when pressed alone)",
        "manipulators": [
            {
                "from": {
                    "key_code": "caps_lock",
                    "modifiers": {
                        "optional": [
                            "any"
                        ]
                    }
                },
                "to": [
                    {
                        "key_code": "left_shift",
                        "modifiers": [
                            "left_command",
                            "left_control",
                            "left_option"
                        ]
                    }
                ],
                "to_if_alone": [
                    {
                        "key_code": "escape"
                    }
                ],
                "type": "basic"
            }
        ]
    }
]