Recently added to the
page by @HulaHoop… Great find!
Depending on your situation you are advised to shut off your speakers and microphone at all times as newer methods of ad tracking can link multiple devices via ultrasound covert channels. This works by playing a unique sound inaudible to human ears which is picked up by the microphones of untrusted devices - deanonymizing you completely. Watermarked audible sounds are equally dangerous. So hardware incapable of ultrasound is ineffective protection. To decrease risks its recommended to play video/audio from untrusted sources with headphones connected and adjusted at a low volume.   
(Text amended by me for watermarked audible sounds.)
More comes to my mind…
Every camera’s sensor has a unique noise signature because of subtle hardware differences.
[speculation] Every microphone has a unique noise signature because of subtle hardware differences. - Plausible?
This is a variation of an older attack perfected during the Cold War where recording typewritter sounds gives enough information to accurately reconstruct what was typed. This still applies today and you should avoid typing in places where open mics are used.
So we should also advice to move all phones, tablets etc. out of the room to avoid them issuing watermarked sounds as well as listening to keystroke sounds and watermarked sounds?
You will probably want to avoid phones altogether and use tablets but
for most situations they are a reasonable choice. Buy a new Android
phone with cash if possible. Avoid other choices because proprietary
operating system is a non starter. You MUST flash a freedom and privacy
respecting ROM before using your camera. Beware that the corporate
malware infestation that comes with the phone out of the box siphons
your data to the cloud aka spy heaven.
I agree in theory, however I am not aware of any alternative firmware that can be downloaded over https, let alone provides gpg verification, not to speak of deterministic builds. Even alternative firmware often contains google components. So it does not look like anyone can effectively adhere that advice in practice.
The most promising android like project I am aware of is replicant, but hardware support is lacking. Not a single supported replicant phone supports wifi.
Apart from replicant, I am not any security focused / privacy respecting mobile devices / mobile operating systems.