Issue: I have just tried the kitten “voice” with the expectation that it would basically
sound like a super high-pitched version of the alto voice. (like as if a cat were talking)
Instead, it makes a bunch of meows.

Suggestion:
1. I recommend making it a language of its own instead.
Maybe even calling it the meow language or … scratch language, I don't know.

2. Maybe keeping a kitten voice but basically making it a cat pitched alto instead of going meow…

Reason:
Because it doesn't affect voice pitch and is exactly the same no matter what language you choose, it certainly isn't a voice.

Summary:
It just doesn't make sense to call something a voice if it doesn't mutate along with the chosen language.
Like it could've went ‘nya’ if I selected Japanese as it would go ‘meow’ in English.

Footer:
Just a thought. If anyone has a reason to disagree with this, I would like to hear why. Thanks.

Support, the tts voices are limited and I’d prefer the meow thing be “lolcat” language like in Minecraft.

dont support tbh, I love the meow voice sm no matter how useless it is

I'm not saying get rid of it. I'm just saying move it to the language option calling it whatever you like.

If you mean an actual language on the website, the thing that Scratch uses for translations doesn't allow fake languages. Though there actually happened to be a meow language before, as well as an Edible Scratch language before 3.0:

I'm pretty sure that the Text to Speech extension has a different list compared to the Scratch website language list.
From the Text to Speech Extension you have these 3 blocks:

 | speak [ hello ]

 | set voice to ( alto v)

 | set language to ( English v)

The set voice to… block contains voices including one called kitten. The set language to… block contains a bunch of languages that the voices speak with. All excepting the kitten voice. Thus, I think the kitten voice should be a language option instead .