Could be relevant:
https://www.reddit.com/r/ShieldAndroidTV/comments/ehbmyr/protip_mute_button_on_2019_shield_remote/

Apparently on the Shield TV you can mute by pressing Volume Up and Down simultaneously.
Maybe that's a feature more Android TV devices share?

Philips TVs have a mute button. TCL also has the function on the remote as the first google result shows: https://imgur.com/a/FmqPgfG

If there is a button you don’t use, you can probably use the Button mapper app. I changed my Netflix button to mute on my Shield. Although you can also press the volume up and down keys to mute. I just kept hitting the Netflix button by accident and it got annoying

The one made by Google has one.

Can't speak for the tvs that have it built in though. I don't use the smart functions of newer tvs and stick to boxes.

TV sets with Android TV built-in typically have a mute button. Sony and Hisense do, at least.

Onn 4K streamer and TiVo 4K streamer both have mute buttons.

My Hisense 55" U8G Android TV has a mute button. I wasn't aware that any Android TV doesn't have one until your info about TCL's. That seems very odd - wonder if it is there and you just missed seeing it. AFAIK the button is typically if not universally a speaker with a line through it and a simple toggle for sound.

The Mi stick (and TV) have a feature where you double click Vol Down to mute. It appeared around six months ago with an update.

My TCL TV has one, and I’d be screwed if it didn’t because when I’m casting from the YouTube app it’ll occasionally bump the volume up to 100 which is incredibly annoying (had my TV over a year and still no fix from Google) but yeah a mute button is a pretty essential feature

Built-in Android TVs usually do, streaming devices with Android TV may or may not

It's Android TV, so you can always make a button with custom behaviors, like double clicking, to perform mute function by using Button Mapper app.