Update error message for disallowed username characters

[Marc4]

I suggest replacing “illegal characters” with “unallowed characters” to avoid unnecessarily alarming the user and to be more specific.

This error message appears when you try to create a username that contains “special characters.”

This is the PR: ​https://github.com/WordPress/wordpress-develop/pull/11590

[Marc4]

Typo error, sorry. I suggest replacing illegal characters with disallowed characters.

[nikunj8866]

@Marc4, please add this ticket link to the PR.

This PR replaces illegal characters with disallowed characters, removing unnecessarily alarmist language for the user.

https://core.trac.wordpress.org/ticket/65084

[Marc4]

Done. Thank's for your follow-up @nikunj8866

[jigarkahar]

Test Report

Description

❌ This report validates whether the indicated patch not works as expected.

Patch tested: ​https://github.com/WordPress/wordpress-develop/pull/11590

Environment

• WordPress: 7.1-alpha-20260416.170638
• PHP: 7.4.33
• Server: PHP.wasm
• Database: WP_SQLite_Driver (Server: 8.0.38 / Client: 3.51.0)
• Browser: Chrome 147.0.0.0
• OS: Windows 10/11
• Theme: Twenty Twenty-Five 1.4
• MU Plugins: None activated
• Plugins:
  • Test Reports 1.2.1

Steps to Test

1: Go to WordPress admin panel (/wp-admin)
2: Navigate to Users → Add New
3: Enter username:MarÇ
4: Fill required fields
5: Click Add New User

Actual Results

1. ❌ Issue notresolved with patch.
2. Still this is same message. not changed.

Supplemental Artifacts

​https://tinyurl.com/29vhvnfl

Tested in WP playground issue is not resolved

#### Trac ticket: https://core.trac.wordpress.org/ticket/65084

---

### Summary

This PR replaces the "illegal characters" with "unallowed characters" across all the occurences for consistency.

@dhruvang21 I have created a new PR for this issue, it replaces all the occurrences of illegal characters with disallowed characters
PR link: ​https://github.com/WordPress/wordpress-develop/pull/11668

[shreyasikhar26]

Created another PR here: ​https://github.com/WordPress/wordpress-develop/pull/11668

Since earlier PR was missing some cases, my PR will be covering it.

@shreyasikhar you have used unallowed keyword instead of disallowed in the message

[shreyasikhar26]

Thanks @dhruvang21, I have updated the same PR by replacing unallowed with disallowed.

[yusufmudagal]

Tested PR ​https://github.com/WordPress/wordpress-develop/pull/11668

Before the patch, invalid username messages used "illegal characters" in both register_new_user() and WP_REST_Users_Controller::check_username().
After the patch, both now say "disallowed characters".

Patch tests well for me.