They don't advertise it at all, but the official website has an atom feed at https://www.nomanssky.com/feed/ which you can subscribe to from an rss reader/news reader. Recent entries include expedition announcements and bugfixes.
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This is what I'm looking for, thanks!
Man it somehow didn't occur to me they might use good ol' RSS. Now I have reason to respect Hello even more...
Look out for updates, generally a new expedition starts some days after one is released, on the official website.
Steam offer a way to keep track of updates (even experimental and internal builds)
Keep an eye on Seam Murray Twitter account, it seems to be a tradition that once he post an emoji then a new update is somewhat imminent.
Look into a service called IFTTT (https://ifttt.com/)
You should be able to configure it to trigger an email when the word Expedition gets mentioned on Twitter, Reddit, etc. and it's free.
Disclaimer: I haven't used IFTTT in ages, so this is all in theory, but it should be possible/trivial. Let me know if it works for you!
Aside from IFTTT going paid, it'd be tough wading through every mention to spot just the legit notice of an upcoming expedition.
I do not think it is free any longer, especially when coupled with Twitter.
No, I don't know. Sorry
How did you know that no one else would possibly know this?