You can configure things like this globally and/or for each method call.

Eg;

    # turn off all runtime checks
    T::Configuration.default_checked_level = :never

    # turn off runtime checks for one method
    sig {returns(String).checked(:never)}
    def foo; :wont-raise; end

Personally if I were authoring a gem I'd leave the runtime checks on except in hot paths, so my users get quick feedback when they pass the wrong thing.

In any case, the library author can get the benefits of static and runtime typing, and their users will get nice static typing if they use sorbet. Users also get nice runtime typing for the library if the author chooses to leave it on for them. The overhead is usually small.

    module T
       module Sig
         def sig *args
         end
       end
       # You'd need to stub out a few more things here.
    end

    begin
      require 'sorbet-runtime'
    rescue LoadError
    end