Petitions asking for an exclusive game to be ported to another platform are not really uncommon, but we don't recall seeing petitions asking for a game NOT to be ported.
Nonetheless, Bloodborne is now subject to a petition-war between two opposite sites lobbying for and against porting it to PC.
With 28,248 supporters at the time of writing, the first petition asks From Software to port Bloodborne to PC, arguing that "everyone should be able to play your game. It is not fair that we are left out. You have betrayed us after releasing your last two games on PC."
On the other hand, the second petition asks the studio no to port the game to PC. It has only 167 supports until now and its message is a bit blunt: "PC gamers are bunch of elitist douche bags. They don't deserve this PS4 exclusive. If they want to play Bloodborne they should buy a PS4 like gentleman (SIC)."
Interestingly enough, both petitions are pretty useless as From Software developed the game for Sony Computer Entertainment Japan which retains the rights to the game and the whole franchise. Needless to say, Bloodborne will remain exclusive to Sony platforms for the foreseeable future.