With ActivityPub protocol being accepted as a W3C standard, it would be a good idea to have it in the old reader.
ActivityPub is implemented in a range of softwares like Mastodon, Pleroma, PeerTube, Funkwhale, Misskey and Plume to name a few. More implementations are coming and it means friends on Mastodon or any of those softwares could follow my profile in the old reader for instance.

There's some explanations on implementation details over here :
https://blog.joinmastodon.org/2018/06/how-to-implement-a-basic-activitypub-server/
And here :
https://blog.joinmastodon.org/2018/07/how-to-make-friends-and-verify-requests/

And the w3c specs are here:

https://www.w3.org/TR/activitypub/

It would be very nice to have more friends being able to follow what I share using the old reader, even ones that don't use a feed reader at the moment.