Ah, okay.
That might be a bit of a rehash, but the current situation is that Serendipity expects your readers to subscribe to your blog with a feedreader.
https://aboutfeeds.com/ for example explains that, and I recently implemented a feature that will show a short explanation and link to such an article directly when visiting the feed with a web browser (so that would be in the next release if not removed again in the future).
One relatively popular alternative in my circle is mastodon. You can use a service like
https://mastofeed.org/ to post your new feed entries automatically to a mastodon account, and readers can subscribe to that account. So that's a twitter like alternative to using a feedreader, and gives you the option to interact with readers outside the blog.
https://kirche.social/explore might be a fitting instance for your site - seems to be mainly german, but is tagged as also allowing english posts.
I present both options in my sidebar like this:

- Screenshot 2025-09-16 at 16-46-03 onli blogging.png (8.42 KiB) Viewed 34539 times
The RSS icon on the left goes to the feed, the Mastodon icon to my mastodon account, "Blog abonnieren" => "Subscribe to this blog".
If I remember correctly what you was searching was a way to subscribe by email to new posts. I still see the problem there that this would constitute as a newsletter, which many or at least some hosters forbid - and this was the functionality a different developer proposed for Serendipity but could not be integrated because of development details. You could instead point your readers to a service like
https://blogtrottr.com/ that uses your existing feed to send emails instead - it's free (with ads) or cheap (without) for your readers, though I don't see a widget you could embed in the blog directly.
The github issue to either correct the message about emailing subscribers or to implement the feature is still open, but a dev would need to come forward to do that work. I might do so myself at some point, maybe it can be done with reasonable effort as a plugin, but with doubts about offering the functionality it's hard to find the time.
As a side note, if the discussion between your readers is that important to you, a discussion forum outside of the blog - or even instead? - might be exactly right for you, independent of s9y having email subscriptions for new entries or not.