It just means that Spectacle leaves the KDE Gear release schedule and adopts Plasma’s release schedule. So there will be Spectacle 6.4.0, 6.5.0, etc. It remains a standalone app, like other apps on the Plasma release schedule such as Discover and System Settings.
This was done to sync up Spectacle with the major software libraries it depends on (KPipeWire and KWin) which also use the Plasma release schedule. Not matching has been a source of bugs in the past.