Select Menu: Tracks
From Audacity Development Manual
These commands extend the current selection up and/or down into all tracks, or Sync-Locked tracks in the project.
In All Tracks Ctrl + Shift + K
Extends the current selection up and/or down into all tracks in the project.
In All Sync-Locked Tracks Ctrl + Shift + Y
Extends the current selection up and/or down into all Sync-Locked tracks in the currently selected track group.