You can literally arbitrarily associate any number to anything and ban it based on that, which they did:
> In 2012, it was reported that the numbers 89, 6, and 4 each became banned search terms on search engines in China, because of the date (1989-06-04) of the June Fourth Massacre in Tiananmen Square.
> Due to the association with gangs, in 2012 a school district in Colorado banned the wearing of jerseys that bore the numbers 18, 14, or 13 (or the reverse, 81, 41, or 31).
> In 2017, far-right Slovak politician Marian Kotleba was criminally charged for donating 1,488 euro to a charity.
It's closely related to hate speech laws where the only requirement is perceived threat. Fun times. :D
Banning specific representations of an idea is a game of whack-a-mole. As soon as you ban one symbolic representation, a new one will be created.
https://en.wikipedia.org/wiki/Chilling_effect
The website dedicated to the topic of the OP article is even called... http://chillingeffects.org/ (since renamed to https://lumendatabase.org/ )
There are no illegal numbers. But if you provide context, they can become illegal data. Let's take child pornography for an example, because that's universally accepted as illegal. I could come up with a formula which translates a huge number into a bitmap. I could post the number (i.e. the input for the formula) online, because it is absolutely meaningless to anyone else. But when I publish my formula, it is no longer a number. It's data which can be interpreted as something meaningful, an image.
Everything is just a number without context.