There's no link to when the decision was made and the history is younger than it.
Working on: Author Appeal | Sandbox | Troper WallIt seems like it might have either been made before Eddie stopped purging old page histories/forum threads, or was done unilaterally (less likely IMO).
Works That Require Cleanup of Complaining | Troper WallThe decision was made between 07 November
and 08 December
2012. The reason is that the examples tend to spiral out of control.
I wasn't able to find anything in the archived discussion pages or forum threads about how the decision was made.
Ukrainian Red CrossI was on the side of making the trope DO at the time, on the expectation that wicks would also be cut. But now that the trope is more clearly defined and we have a proper cleanup going, it should be feasible to open it up to examples again.
Stories don't tell us monsters exist; we knew that already. They show us that monsters can be trademarked and milked for years.
Isn't that already the definition of Aborted Arc ?
Not really. There's the difference between "setting something up but ultimately going down a different path" and "completely dropping something".
Besides, Aborted Arc is for, well... arcs.
Working on: Author Appeal | Sandbox | Troper Wall

Why is it that They Wasted a Perfectly Good Plot doesn't allow on-page examples? It feels like a pretty arbitrary decision, especially given that They Wasted a Perfectly Good Character allows them.
The on-page disclaimer reads:
But I don't get this reason. There actually isn't that much overlap with Fan-Preferred Cut Content, and "it's a matter of personal tastes" is something that applies to all YMMV, yet others accept on-page examples just fine.
The no on-page disclaimer has been there since at least 2013
but I can't find what exactly prompted this decision.