Peter, https://www.subtome.com AFAIK just redirects you to a RSS/Atom reader and such buttons exist for ages.
There's a little more to that. subtome could include h-feed parsers as options as well.
RSS is dying and I prefer #IndieWeb h-feed because it's part of web-site instead of an invisible XML-addition. RSS is dying for an simple reason — it's one-directional publisher->subscriber technology. With rise of social networks with their #P2P communications RSS is really "walking dead" if we talk about human-to-human communications. RSS still used for one-directional public updates (news, announcements, etc) distribution.
Valid reasoning, and I'm not going to deny the truth of it. However, for me, a reacji is not interaction - which is currently rising on the silos - so while the option of interaction is important, the content of these reactions are becoming less and less relevant and significant.
The 'Subscribe' button I miss should allow to subscribe to an private h-feed (like family or company feed) and allow bidirectional communication: I should be able to send comments to feed's owner and receive replies back.
I totally agree that we need a solution for the missing Follow/Subscribe option. However, I think this is more of a missing service/infrastructure than an actual missing #indieweb feature.
Similarly to this, my greatest itch is content discovery1 as I'm dealing with RSS & Mf2 by sending them as mail from a list of URLs as subscribed for sites. To think of it, it may not be impossible to add a little glue system that handles reply email for those posts sent as one and let my site post them as actual replies...
You've just given me an idea, thank you!