In version 8.0 we introduced new 'PCB Interaction' preference “Delete Track Does Unroute” (default setting checked) to prevent users from unintentionally deleting connections on the PCB side of a project.
Now, when users select a segment of track and tap the 'Delete' key only that specific segment of the track will be deleted. The connection between the two remaining parts of the track will still be visible if there is a net (described by net-list) between components being connected by that track.
You can still remove the remaining visible connection by placing the cursor on that connection and once again tapping the key 'Delete'. Deleted connections will become invisible, but if there is a net between remaining parts of the track it will still exist (but invisible).
You can make the connection (and the net) visible again by selecting the option "Optimise Nets":
Right click on selected track -> Net -> Optimise Nets
Alternatively you can find the option in: Main Menu -> Tools -> Optimise Nets
(Shortcut Key: Ctrl + D)
Should you decide that this functionality is not ideal for your project you can turn it off (and comeback to previous releases default routing behaviour) simply by unticking ‘Delete Track Does Unroute’
Main menu -> Settings -> Preferences -> Interaction