Daniel Lange's blog

Updating to irssi 0.8.19 (which is a mainly a bugfix release to 0.8.18) proved a real issue. The enter key (return key) stopped working. Ctrl-J still worked but that's way too annoying to remember after each line. Searching the github issues turned up #327 Numeric keypad "Enter" key stopped working which didn't help much. Digging deeper it shows the irssi devs enabled "App key" mode in these releases which causes so many issues, they had to implement a switch to turn it off again.

So a hopeful: /set term_appkey_mode off followed by Ctrl+J, remember ...

and ... nothing changed.

So finally, after more digging and a quick consideration to go back to irssi 0.8.17 on Debian stable (Jessie) ...

/bind ^M key return

Yes, irssi 0.8.19 wants to be told what the enter key is, like, by default. Duh.
No idea what caused this in my configuration, I've been using irssi for more than a decade so much cruft has accumulated in my .irssi/config but ... in case you run into this as well, hopefully I helped you save a morning for something better to do.

If you want to fumble this into your .irssi/config (e.g. because Ctrl-J does not work for you):

keyboard = (
  [...]
  { key = "^M"; id = "key"; data = "return"; },
  [...]
);