I did a test and found out this:
If i re-install Premiere Pro and open the Keyboad Shortcuts dialog i can see Timelines as an option in a new empty project and can see that the current command is Shift+3. If i then import one (1) clip into the project, create a timeline, place the media on it and open the Keyboard Shortcuts dialog the option to re-map Timelines are gone.
Bottom line:
Once one or more timelines are open in the timeline pane the option to manually re-map the keyboard shortcut disappears from the Keyboard Shortcuts dialog. Close all open timelines and the option should be back again in the Keyboard Shortcuts dialog. But, you can not map it to any key. I can map the alpha numeric 1 on to open the Project panel. I cannot any more map the alpha numeric 3 to open the Timeline. The the alpha numeric 3 worked on all Pre CC2017 versions and i have used that key for years. (I can map the alpha numeric 3, but it dont work for the Timeline.)
However, i am still is surprised that such a bug that has proven to be so easy to find left the beta phase. I can´t even use the keyboard shortcut for Extract without re-mapping it. I use Extract many, many times per/day...