Login Name Restrictions

Good afternoon. In this article, I would like to talk about the restrictions that are set on services for logins.

When creating another service, I proceed from the following considerations:
1. The login should not be too short (I now have 3 characters or more, for larger systems, such as GMail, I’ll use 6 or more).
2. The login should not be too long , otherwise in some interface it will start to burst something incorrectly. I use logins up to 20 characters, but it is interesting to hear your opinions on this matter.
3. The login can be used in the address bar, for integration with SMS and so on. Therefore, we leave only Latin characters, numbers, underscores, hyphens and periods.
3a. If the login can be used as the name of a subdomain , then these conditions are reduced to Latin characters, numbers and hyphens (a hyphen cannot be the first and last, it cannot be repeated).
4. Sometimes it is very convenient to use dual navigation: by login and ID. For example, we use the address domain.com/users/ on one service for users with login and domain.com/users/for those who do not have a login (so-called virtual profiles that are registered by the administration or other users). This leads to the fact that the login cannot consist only of numbers , so as not to confuse it with the identifier.
5. Exception words : www, mail, ns1, ns2, ns3, ns4, admin, god, moderator, editor. The first 6 are exceptions for compatibility with subdomains. What exception words do you use?

And the main question for the readers: what limitations do you have and what have I forgotten?