It's the same story on all servers.
There are tons of suspicious spots that are easily reachable via snowfall or jumps and that give you a strategic position to help you win the game.
Obviously some of them are not a big deal but you also have spots that give a huge advantage. For example you can get completely off map with flag in warsong - without speed item or engineering or mage blink.
If I was to FC against some of the people bashing on OP, and get the flag to some funny spot, they'd probably make a similar thread to this one. Even though all their argument here would stand for my funny spot.
So it's very hard to give a line not to cross, or to give a list that are acceptable or not.
Most likely, wethere it's allowed or not will depend on the GM that will handle the case. And of course, if a lot of people get butthurt about one specific spot, it will be more likely to be forbidden in the future.
To be short, I don't think you'll have an official answer on wether it's allowed or not. If you want to do it, it's at your own risk. If you got butthurt by someone doing it, report it ingame and hope to get the right gm to answer it.
As to know if any of those spot are "abuse". Well, they all come down to the map having flaws. Those flaws are blizzlike. But we don't know if they were intended by Blizzard. Probably some of them were and some weren't. So you can't really know for a specific one if it's an abuse or not.