Fix: Mark trait as experimental

[localheinz]

What is the reason for this PR?

• [x] marks a trait that depends on and is solely used by experimental classes as experimental

Follows #154 and #165.

Author's checklist

• [x] Follow the Contribution Guide

Summary of changes

Review checklist

• [x] All checks have passed
• [ ] Changes are added to the CHANGELOG.md
• [ ] Changes are approved by maintainer

[GrahamCampbell]

Kinda too late isn't it? It's already in existing releases?

[localheinz]

Closing in favour of focussing on 2.0.0.