What rules has my username broken?

I've had this username forever. You force me to change it you will take away my only reason to play on this site. 

The site is changing it so usernames can't be just numbers.

That sucks. Why?

What's your username? I can't find it.

I can't speak to the specifics but there's a reason for the change and the username will automatically be changed if not completed by the member.

Can we at least get the parameters for acceptable usernames?

"Your username must be at least 3 characters long and can only include letters, numbers, underscores, and dashes. It must start and end with a letter or number. Spaces are not allowed."

OK, I tried "ELO-314158265358" and it said "Username is not allowed. Please try again. " Why? Too long? Chess.com hates Electric Light Orchestra? Bunch of cake eating pie haters?

The above doesn't even say the max length a username can be.

As far as I'm aware, that should have worked.

let him keep his username 💀

There's a reason for the change snd as far as I'm aware, no number only username is going to be retained

Just replace the 0 with an O.

Very nice to a) not mention the actual rule while asking to change username b) acting like this is 1998 yahoo.com where you have un-sanitized inputs to your database so you have to declare all usernames with just numbers illegal. and c) Making this your focus while there are several other actual problems to be fixed on this website. 
What would the reason be if I may ask?

@Martin_Stahl who knows the specifics?

Only staff can say why but I don't know if they will release more specifics.

@404 I do know it's nothing to do with un-sanitized inputs.

They could at least let us know the maximum length. If certain words are censored, like semprini. Because usernames can be changes can we use one someone else used to use, or is the username a seperate field from the account or not necessarily unique (i assume it has to be unique)

Testing new name.

Max length is 25 characters. For possible censored options, that's something you'll need to look at and see if you can see anything in the name that could be considered a vulgar term, profanity, etc; e.g. a username that has something like that as part of the name won't be allowed.

As to existing usernames, that would follow the requirements under https://support.chess.com/en/articles/8599051-how-do-i-request-an-inactive-username