TeamCity, YouTrack Standalone, Upsource, and Hub remain under the perpetual licensing model. These licenses are perpetual for the versions they cover.
This means that you can run a covered version with existing licenses for unlimited time, and the licenses will stay valid for this version.
Each license has a maintenance period (generally 1 year). The license key is valid with any version released within the maintenance period. Licenses valid for the major/minor release (changes in the first two release numbers) are also considered valid for the corresponding bug-fix updates (changes in the third release number).
YouTrack inCloud, a hosted solution by JetBrains, has been and will continue to offer subscription scheme for the service.