I'm not going to lie: I used the stacking buff to get things up to level 25 to sell.... but not in the exploit way. Just 1 hat + 1 sausage + 1 burger + 1 pet weekend and/or Squirt.
What with how fast and easy tokens are to get nowadays, though, honestly leveled pets *are* mostly devalued. Considering it's only 45 tokens to get a level 25 pet like that.
.... which is sort of funny, considering that for 25 +1 stones it costs 75 charms.
.... which is off topic, admittedly. My point being that I don't think they're going to make them stack again, simply because it's already very easy to level pets, and so stacking 5% and 10% pet XP isn't necessary and, if it's being exploited, is just better off removed altogether for ease of coding. At least that's my guess here. :/
It would be super nice if they made them stackable again, but I strongly suspect that they originally weren't meant to be stackable, and they just went "Well, it's a minor thing," ..... until it wasn't.