According to the Facebook spokesperson quoted in this Marketing Land article:
We are currently testing the ability to subscribe to Pages with a
small group. This feature allows people to receive updates from Pages
without liking the Page. We have no further details to share at this
Subscribers can get your public posts in their news feed. However, friend requests are a two-way interaction with more accessibility.
The friend requests that were sent before enabling subscription would not be automatically added as subscribers.
Subscriptions are only for the requests after enabling the feature.
It was my experience when I enabled subscriptions on my account that friend requests and subscriptions are completely separate once they're enabled. They were designed to be separate, with subscriptions being a way to filter what shows from a profile; a friend can subscribe and use that subscription to filter what gets into his feeds from your profile, and a ...
This includes a lot of miscellaneous things that may appear in the News Feed but do not fit in the other categories, such as the person posting to or being added to a group, attending an event, getting new Facebook friends, changing their profile information, being tagged in a non-photo post, or anything from one of their Facebook apps that is not classified ...
By default you will see anything from that person that is publicly visible, including public comments. To turn these off, go to their profile or timeline and find the "Subscribed" button at the top, or hover over their name in one of their posts. While hovering over the "Subscribed" button, go down to "Comments and Likes" and uncheck it to stop seeing ...
According to Facebook Support your friends will see when you subscribe to a page's events. You may want to traverse through your profile's privacy options to see if there is a way to disable that sharing.
Go to their profile page (Timeline) and select Friends → Settings. You can then select what updates you want to see (uncheck all options to get no updates). You can also select Only Important to change the frequency of posts that appear in your newsfeed, in case you don't want to unsubscribe from all posts.
As of current Facebook's policy(and trying out myself):
"provided that their requests are still pending"
A Friend request is never automatically made into a subscription request or made as a subscriber. Regardless of the period the friend requests stays pending, they remains to be "pending friend requests". But meanwhile they them-self can subscribe to you....