[Suggest] Warning message to migrate to terser when harmony option is true

[kimdj2]

The harmony project in UglifyJS has been without updates for a long time.
Can you warn to use terser when compiling with the harmony option set to true?

Many rails guides recommend using terser as a compression for js.
I also know it's already mentioned in the README.
However, since the uglifier gem is still heavily used, I want to encourage them to use terser for compressing ES6.

Recently, I left a problem with UglifyJS due to a bug in ES6 syntax in graphiql-rails in harmony mode, but hamory was no longer in a state to be updated.
rmosolgo/graphiql-rails#106 (comment)
mishoo/UglifyJS#5823

I'm sure there will be more issues like this in the future.
I would appreciate your confirmation.

[johnkw]

It would be very helpful if this entire project was marked as abandoned on the github home page. The help output should also be updated to indicate users should migrate to something else.