What minimal example or steps are needed to reproduce the bug?

::picker(select) {
  color: red;
}

select::picker-icon {
  color: red;
}

What minimal configuration is needed to reproduce the bug?

{
  "rules": {
    "selector-pseudo-element-no-unknown": true
  }
}

How did you run Stylelint?

Demo

Which Stylelint-related dependencies are you using?

Stylelint wrapped in @wordpress/scripts v30.15.0.

What did you expect to happen?

No problems to be reported.

What actually happened?

The following problems were reported:

example.css
1:1-9  ✖ Unexpected unknown pseudo-element selector "::picker"      (selector-pseudo-element-no-unknown)
5:7-20 ✖ Unexpected unknown pseudo-element selector "::picker-icon" (selector-pseudo-element-no-unknown)

Do you have a proposal to fix the bug?

Add both names to Stylelint.
Edit: PR created: #8623.