There are 2 distribution lines of Scala 3: Scala Next and Scala LTS.
The Scala Next line is the default to be used by most users, containing the latest features, bug fixes and improvements.
The Scala LTS line is advised to be used for publishing libraries. (Some especially conservative users might also choose it over Scala Next.)
Bug fixes and usability improvements from the Scala Next line are regularly backported to the currently supported LTS lines.
Scala minor releases (3.x) are backwards compatible, meaning libraries built with older versions can be consumed by newer versions. Patch releases (3.5.x) are backwards and forwards compatible.
Thus, existing source code continues to compile when upgrading to a new version, except for rare situations when a bugfix requires breaking source compatibility.
Scala 2 TL;DR
Maintenance of Scala 2.13 will continue indefinitely.
Minimal maintenance of Scala 2.12 will continue as long as sbt 1 remains in wide use.
Maintenance of Scala 2.13 will continue indefinitely.
Minimal maintenance of Scala 2.12 will continue as long as sbt 1 remains in wide use.