..which you can turn off or disable..

Can you?

No matter how many damn Google Assistant settings I disable in how many fractal menus, it STILL "helpfully" pops up if I say out loud "ok google"!

Like, "heyy I noticed you're trying to talk to me though I'm disabled, would you like to enable me?"

Stop spying on me you piece of malware.

What kind of Android do you have? Are you sure you went through all the settings?

And, still, I can install LineageOS or other privacy-respecting systems on my phone. We don't have any custom firmware for an Echo or Alexa yet (to the best of my knowledge.)

The number of phones actually supported by lineageOS is a rounding error, unless you count unofficial incomplete builds by random untrusted people on some forum.

Disabling Google Assistant was trivial on my Android One/Google Fi phone. It was one of the first things I did.

Almost none of my family members understand the implications of the voice assistant let alone know how to turn it off. You’re a techy type and are thinking my uncles or grandparents operate at your level.

What fraction of iPhone users do you think have turned off Siri? Based on my circle, it's zero. Android seems a bit more complicated because registration for Samsung or Amazon might be decoupled from device setup, but I'd imagine those are mostly turned on. So most people do have an always or nearly always on microphone in their pocket as the post higher up was trying to point out.

Anecdata, but most of my friends that have iPhones has Siri enabled indeed, but the minority that is aware of the "Hey Siri" feature (the only thing that needs an always-on mic) has not enabled it. Most of them find it creepy.

Not sure if these are just uneducated posts or astro-turfing...

Astro-turfing for what? I guess this falls into the "uneducated" category. From my apparently uneducated perspective, phones need to be a concern if you're worried about your voice and activities being recorded. Most users have no idea that, for example, Google Assistant is always listening on Pixel.

Siri is not enabled by default, they use a different approach to get users to opt in. Step 8: https://support.apple.com/en-us/HT202033 has a much bigger button to enable Siri, but users are given a choice and skipping Siri takes less time so it’s easier if you’re not going to use it.

Completely deactivate it

If you choose to disable Google Assistant completely, follow these steps:

    Open the Google app and select Menu (the three bars)
    go to Settings
    Google Assistant section click Settings > Phone and disable the Assistant option

I have a custom domain and disabled it for my family:

Switch off Google Assistant (for entire custom domain) Click Search and Assistant. Tip: Scroll to see all services. ... Control who uses Search and Assistant in your organization

    At the left, select the organizational unit.
    Select On or Off.
    To keep the service turned on or off even when the service is turned on or off for the parent organizational unit, click Override.