Problem/Motivation
In #3215045: [Duplicate] Highlight obsolete module's status at admin/reports/status page, providing warning and link with explanation we discovered that there were 2 themes who used experimental: true instead of lifecycle: experimental in their .info.yml. We're changing that in issue #3250585: Highlight deprecated modules and themes at admin/reports/status page, providing warning and link with explanation.
However unlikely: There could be Contrib out there who also use experimental: true therefore we need a nice deprecation for this.
Exactly for that this issue was opened.