Lol. It's indeed funny and weird.
What's really interesting is that after trying to find something useful on the proboards forums, I keep stumbling upon stuff like this:
Hi.
Unfortunately, you can not embed documents into a post, as in they display in a post. You can only link to a document at this time, sorry.
From
this thread which in fact is only a couple months old.
Mysterious: the proboards people claim it's impossible, yet we have more than a few examples of it working properly on our own boards, and still viewable (I'm guessing that's what brought you here?...I noticed that Bad's post/this thread returned as a top result for google queries involving proboards embed google doc spreadsheet etc etc).
Maybe someone should ask proboards why it worked on our forums and the old embedded docs are still visible but it can no longer be replicated or reproduced...? Or maybe this is all very simple and obvious to someone with coding knowledge and the answer is surprisingly mundane...shrug.
From what little research I did, and what little I understand of it, it looks like
a) google docs setup had changed, involving "google drive" in place of the older system
(and maybe this impacted what ubbcode worked now)
b) it looks like a forums admin could have constructed the functionality for this via whatever proboards scripts/plugin etc
(yeah, really clueless about that tbh, but I really doubt anyone spent time to implement this here...maybe that's incorrect though). I was reading about this "iframe" thing and plugins and Google Drive and cloud and blahblah, but I don't get why the old ones here on our forums still view properly if it shouldn't be possible...
I know that Bad made a bunch of posts with embedded google sheets and iirc I saw at least one or two others do this as well.
I tried quoting a post and re-using the exact same code (such as OP), but it doesn't work, as you also discovered. We can clearly see that Bad has the code which evidently "worked then" and is visible to us now, yet cannot be replicated.
Why is it "still" visible and properly-embedded for old posts but not usable anymore?
Maybe proboards deliberately restricted this functionality...?
I'm guessing it's probably still doable if you figure out how...good luck....
Edit: Well I just found something interesting...when going back to an old post with embedded doc, it still shows properly. Like
highergroundpoa.proboards.com/thread/22353/big-fail-pariah-barb-sorcBut a few minutes ago I tried to insert a link to Bad's post with the doc in it. When I tested the link, the "new" post page that opened in my browser did NOT properly show the doc! Gah proboards, why!?