PSG
Barcelona
Man City
United
Chelsea
Real Madrid
Juventus
and many other teams
All of them leak SHIT to the press the way Liverpool does. I don't get why you keep on insisting only Liverpool does this shit when every fucking team does it.
Specifically to what went down with VvD, I don't think it gets that bad all to often.
First, Liverpool tried to win public opinion in March / April by having the club mouthpiece reporters all come out with articles about the 3 or 4 names Liverpool will sign this summer. This part is pretty common, but is usually more a Barca tactic.
The celebrating over reaching an agreement from those same mouthpieces is the odd part. Other clubs tapped VvD up. Any decent club tapped other players up. What most clubs didn't do was publicly celebrate signing a player before even contacting the selling club. This is far more uncommon.
The source matters, too. There are endless tabloids and sites across Europe who will report Coutinho is signed or Can is off to Juventus. But it's usually not from sources recognized as club mouthpieces like James Pearce. At least not in the PL.
So yeah, in a sense everybody does it. But the degree Liverpool took it with VvD is far less common. Barca takes it it to that level all the time, but it's not done run of the mill thing.