Only allow jobs to be at most hourly?

this new option affects only server owners or all cronjobs?
I have ntp running every 20 minutes for instance.

also does it apply to newly created jobs or also the existing jobs.

thank you

It has no impact on root, and has no impact on existing jobs. It also has no impact on jobs created on the command line. It is merely a limit you can impose in the cron jobs UI.