Alas swiftpackageindex.com apparently is still using an older version of DocC as my documentation is still largely missing there, but that'll be fixed once Xcode 15 exits beta, I assume. @daveverwer & @finestructurecan correct me if I'm wrong.
By default we're generating docs with Swift 5.8 but package authors can opt into using 5.9 by specifying it in your .spi.yml file:
version: 1
builder:
configs:
- documentation_targets: [Target1, Target2]
swift_version: 5.9
Hope that helps!