Trait parameter backport to Scala 2

SIP trait parameter is approved for Scala 3. Can we make it backport to Scala 2? It is incredibly useful to ensure initialization order and reduce runtime error and unexpected behavior.

Unlike some other SIPs for Scala 3 (e.g. opaque types can be done with tag, , there is no way to workaround to achieve this feature, so I think it will be really beneficial to backport it.