I have tried to test/spot check this but haven’t been able to confirm either way:
Do globally pinned topics get “pinned” also to the top of Digest emails that go out?
If no, why not? Since they are stuck to the top of topic lists, it seems that they should also be highlighted to make sure they’re seen by digest readers to get their attention, no?
no they do not, I see no reason why they would, maybe an edge case should be allowed where globally pinned topics are on top IF and only IF they were pinned during the week of the digest.
Because a global pin is something you want to make absolutely certain everyone is aware about, no?
If it doesn’t show up in the digest, then we’re not doing all we can to make sure everyone is aware of it.
The equivalent UX in the digest email that is comparable to the web, would be to keep sending it at the top of the digest until either (a) the topic is un-pinned, or (b) the user dismisses the pin, whichever comes first.
Every week for the rest of forever?
No, only until the user dismisses the pin or staff removes the pin. Just like the web site.
Maybe, in 1.4 we have the planned concept of forcing site owners to always specify an expiration date on pins. Only then could this even be considered.
I do quite like the idea of a pin meaning that it definitely goes out in that week/period’s digest (and no other), and it doesn’t require the pin expiry.
We’d love the ability to ensure a topic goes out in the digest for the reasons @downey posted. Whether that’s via the Pinned mechanism or a separate feature.
Please consider this a vote for this feature! 
Oh hey, this exists! Adding custom content to summary emails
This’ll get us what we need for now.