Keyboard shortcut to show Tags... dialog
This no longer seems possible with Mac OS 10.14. My workaround has been to use Keyboard Maestro, with the following macro:

For Each Item in the Collection Execute Actions
    For Each FilePath in the Finder selection
Execute the Following Actions:
    Get Tags to Variable ‘Tags’
    From file: %Variable%FilePath%

Prompt for User Input ‘Tags’
    Prompt: Enter tag to toggle
Input the following variables:
    Tags (default to ‘%Variable%Tags%’)
Finish with the following buttons:
    OK
    Cancel (cancel macro)

For Each Item in the Collection Execute Actions
    For Each FilePath in the Finder selection
Execute the Following Actions:
    Set Tags to ‘%Variable%Tags%’
    File: %Variable%FilePath%


If you select some Finder items and run this macro, it will prompt you to enter tags. You can enter multiple tags separated by commas. Pressing OK will apply those tags to the selected items. The prompt will be pre-filled with the tags of the last item in the selection (this part of the macro isn't ideal).