On Discourse Hosting you are limited to the plugins allowed in your plan. You cannot install plugins on your own and on most plans you cannot even get custom plugins. This is limited to enterprise plan. On other plans you can only use those already installed.
At Discourse pricing | Discourse - Civilized Discussion you can see the plugins included in each plan.
The plugins included in the free plan are very limited
What is your use case for the restricted-replies plugin? Maybe there is a different solution to that problem.