Why can't I use certain characters?

For example, I tried to type some upside down text onto the forums, and it said that I couldn't use the upside down A, upside down Y, and upside down T because they were somehow invalid characters.

While I can likely survive without upside down text, the problem is potentially worse.

The upside down T that is normally used can also be used to indicate that something is perpendicular, and hence has a use. Removing that makes it harder to express that something is perpendicular more simply, and therefore makes some math questions harder to answer in off topic.

The upside down A also has a mathematical usage, albeit a more complex one.

The upside down Y is considerably rarer when it comes to usage, but can be used for some useful purposes.

Now it may be pointed out that these characters generally are rarely used, but this large removal of characters could lead to more useful characters being removed too. I am thinking this is a potential effect of a mass character ban in order to prevent empty messages, but is it possible for lichess to look into this and/or let me know what has caused this?

@A_0123456
First tell how are you able to type them?
~ Do you have special keyboard that contains such characters?
Also, one can simply copy paste the symbols or the material.

Isn't writing the word "perpendicular" easier than making a topic on it?
Why would you try to write mathematical formulas here? Foolish complaint!

@Akbar2thegreat said in #2:

@A_0123456
First tell how are you able to type them?
~ Do you have special keyboard that contains such characters?
Also, one can simply copy paste the symbols or the material.
Copy-pasting the symbols doesn't work. That's what I mean.

@Sybotes said in #3:

Isn't writing the word "perpendicular" easier than making a topic on it?
Why would you try to write mathematical formulas here? Foolish complaint!

1. I said off topic, in case someone has a related question.
2. "Isn't writing the word "perpendicular" easier than making a topic on it?" That's completely unrelated to this.
3. If you're just going to rebuke at me and say "Foolish complaint!" without concluding properly, that's just rude.

@A_0123456
But how are you able to write those characters which results in invalid character message by Lichess?

@Akbar2thegreat said in #6:

@A_0123456
But how are you able to write those characters which results in invalid character message by Lichess?

I wrote the characters but couldn't post them because when I click "reply" or "send", it just says invalid character

I have no problem writing umop-apisdn.

Seems this isn't the font of all knowledge.

@A_0123456 I can answer the question: Why can't I use certain characters?
That is because they are turned off