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It's a folk superstition that has nothing to do with how messengers actually work. WhatsApp, Telegram, and any other app don't have any filters or restrictions on sending messages with numbers - the code doesn't even distinguish what you're typing.
The superstition comes from a tradition where giving money as a single coin or bill was considered bad luck. People used to think it could lead to quarrels or separation. When this superstition met the internet, people just transferred it to numbers in messages - like, a single digit falls into the same category. But it's pure psychology, nothing more.
Go ahead and send whatever you want. There's no real harm in it, and if someone believes in superstitions and is worried, you can just send two digits next to each other or say something along with the number - and the anxiety disappears.
I've sent plenty of messages with numbers - and nothing bad happened, I gave them without any worries. I think it's just a superstition or some kind of urban legend, because WhatsApp or Telegram themselves don't have any restrictions on those kinds of messages.
It's actually just superstition - there's no technical restriction in messengers whatsoever. It's just that in Russian culture there's a belief that giving a single coin or banknote is considered bad luck - supposedly the giver wishes poverty on the recipient. That's where this superstition about single digits in transfers and messages comes from, though it makes no logical sense. Apps like Telegram or WhatsApp don't have any restrictions on sending messages with any digits in them - it's just folk superstition.
If you're worried about what people think, there's a simple way to get around it - just add something else to the digit, whether it's a heart, a space, or a word. Lots of people do exactly that: they write "100❤️" or "100 rubles" instead of just "100". It looks more natural in the context of a conversation anyway, and if you believe in the superstition - supposedly the rule is about a lonely digit, not a number in a message.
But realistically, you can send any digits without worrying. The superstition lives in people's heads, not in the technology.
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