belongs_to に条件をつけることで、default_scopeをunscopeすることができます
Rails6.0で確認

# 古きよきdeleted_atで論理削除できるとします
class User < ApplicationRecord
  default_scope { where(deleted_at: nil) }
  has_many :articles
  has_many :comments
end

class Article < ApplicationRecord
  # Userが論理削除されていても参照できます
  belongs_to :user, -> { unscope(where: :deleted_at) }
end

class Comment < ApplicationRecord
  belongs_to :user
  # このように、default_scopeをunscopeした関連を定義することもできます
  belongs_to :user_with_deleted, -> { unscope(where: :deleted_at) }, foreign_key: :user_id, class_name: 'User'
end

test "article" do
  user = User.create! nickname: 'nickname'
  user.articles.create! title: 'article_title'
  user.update! deleted_at: Time.zone.now

  article = Article.first
  # userを論理削除した後でも取得できます
  assert_equal user, article.user
end

test "comment user" do
  user = User.create! nickname: 'nickname'
  user.comments.create! text: 'comment'

  comment = Comment.first
  assert user, comment.user

  user.update! deleted_at: Time.zone.now

  comment = Comment.first
  # 論理削除後はnilになります
  assert_nil comment.user
end

test "comment user_with_deleted" do
  user = User.create! nickname: 'nickname'
  user.comments.create! text: 'comment'

  comment = Comment.first
  assert user, comment.user_with_deleted

  user.update! deleted_at: Time.zone.now

  comment = Comment.first
  # user_with_deletedを使えば、論理削除後でも取得できます
  assert user, comment.user_with_deleted
end

has_many等でも同様のことができます
こちらはactiverecordのテストケースから一部抜粋させてもらいました
- https://github.com/rails/rails/blob/main/activerecord/test/cases/associations/has_many_associations_test.rb

class Car < ActiveRecord::Base
  has_many :bulbs
  has_many :all_bulbs, -> { unscope(where: :name) }, class_name: "Bulb"
end

class Bulb < ActiveRecord::Base
  default_scope { where(name: "defaulty") }
  belongs_to :car
end

test "test" do
  car = Car.create!
  bulb1 = Bulb.create! name: "defaulty", car: car
  bulb2 = Bulb.create! name: "other",    car: car

  assert_equal [bulb1], car.bulbs
  assert_equal [bulb1, bulb2], car.all_bulbs.sort_by(&:id)
end