There is no warning before Katello 4.5. I have only noticed it with 4.5 (see original post above).
Correct. As long as you don’t use SCA, you assign subscriptions (aka products) to content hosts and they get access to all repositories in the product. As you cannot set the default for repositories in products to disabled, a content host has all repositories of your custom products enabled by default. (Of course you can hide repositories with content views and you can use activation keys to set most repositories to disabled for new content hosts…)
Once SCA becomes mandatory, there are no subscriptions anymore. Content hosts connected to your server have access to all repositories (visible in the assigned content view), which again are enabled by default for all repositories in all your custom products.
Still, it needs preparation because otherwise your client hosts all of a sudden will have tons of repositories visible and enabled after the change…