While you can deactivate dimensions with all versions of Matomo, if you use Matomo On-Premise and have command line access, it is possible to delete a dimension too. To remove a dimension, visit the Custom Dimensions section with your Matomo settings to confirm the scope and ID of the dimension you would like to delete. You can then log in to your server via SSH and run the following command, making sure to replace the items in capitals with the values of your custom dimension:
./console customdimensions:remove-custom-dimension --scope=SCOPE --index=INDEX
To provide an example, if you wanted to delete the dimension in the screenshot below; you would replace $scope with visit and $index with 1.
So ultimately, using the example above, the full command would look like this:
./console customdimensions:remove-custom-dimension --scope=visit --index=1
Note: You can find more information on using the Matomo Console via command line here.