Razien at your service!
Is 25 twinking even a thing?
It's a thing if you want it to be. There are very, very few of us left. What makes us unique like you said is that we're still the very earliest original level cap, which for the sake of rarity means that it is worth keeping. Same counts for 27s and 30s (70 and 80 respectively).
We're at quite a disadvantage against level 29s however it's manageable. I haven't queued for bgs in a long time because I'm rather busy with other things now but the last time I did, I blasted through most 29s. That said, at the time Warlocks and Hunters were a massive problem and a good healer would be very difficult to kill. The main culprit of that however wasn't really the character levels -- it was just the overtuning of spells like Chaos Bolt and Aimed Shot (+ Double Tap which is removed next patch).
Gear-wise our bis is about the exact same as what you'd find at level 20; full Warforged WoD gear with Socket (and potential tertiaries) on them. Farming this out will take you quite a while, however.
The main problem in the 20-29 bracket is that you can't enchant almost anything anymore due to all old enchants having a cap of Ilvl 50, which blizzard has not fixed yet. This isn't unique to 25s, it's a problem for everyone and goes far beyond just twinks, regular players included.
Currently out highest Ilvl possible (warforged gear) is Item level 100, while for lv29s it, if i recall correctly, is 112.
It's not a large difference in power -- the main difference is health. 25s will have much lower total health than 29s do, about.. 2-3k differences sometimes depending on the class.
PvP aside, personally I've long since resulted to doing a lot of PvE-oriented content as well as collecting old stuff such as crafting patterns and farming out reputations. It's something to do when I'm bored and the more of it I get, the rarer my character becomes.
So, all-in-all..
..You can PvP if you have a full set of Ilvl100 gear, though depending on your class don't expect to absolutely "dominate" each lv29. They definitely have a large advantage however, we're also not a lost cause.
..You can also decide to keep it as a museum character due to its incredibly rare features and do PvE content. It can definitely be a lot of fun!
Furthermore - Next patch they're rescaling a LOT of gear once again, I don't really know where they're going to go with it, it's kind of a blur. They seem like they've temporarily un-done all of the existing scaling including those in old raids that before were scaled to lv30+. This could potentially mean we get our raids back or that at the very least they'll properly scale it (optimistic), which would make us much stronger. It could also just mean nothing because.. Blizzard. We'll have to wait and see.
YEAH HELLO RAZIEN BBY XOXO
Hey buddy <3