Sequel’s STI is not so well documented as ActiveRecord’s is. Because it cost me about 20 minutes to find out about it’s syntax, I post a little example.

class A < Sequel::Model
  plugin :single_table_inheritance, :object_type
end
 
class B < A
end

The second argument is the name of the column to be used to store the class name in DB. Don’t use ‘type’ here, it collides with Ruby’s Object#type.
This works with version 3.5.0 and should work with any version >= 2.12.0.