This is a summary of the article, and some of the code lines are taken from there:

https://www.bigbinary.com/blog/rails-7-adds-endless-ranges-for-activemodel-validations

Code:

# endless range example
class Person
  validates_length_of :first_name, in: 20..
end

# endless range example
class User
  validates_length_of :first_name, within: 20..
end

# endless and beginless range example
class Food
  validates :created_at, inclusion: { in: proc { (..Date.today) } }
  validates :expired_at, inclusion: { in: proc { (Date.today..) } }
end