Traditionalists. They're a difficult group to define because there are so many different factions among them, the most harmless (and biggest, I hope) is the group of people who simply prefer the Tridentine Latin Mass. l
However, it doesn't aways stop there. Certain corners of the Internet have become echo chambers for trashing the direction the Church has gone since the Second Vatican Council as well as the council itself. There's also a lot of gatekeeping among that crowd and posturing about how they're the only "true" and "faithful" Catholics. I pretty much quit browsing certain parts of the Internet because they got overrun with those types: the type of people who unironically call themselves more Catholic than the Pope.
In short, Pope Francis has recently put more restrictions on the pre-Vatican II Mass with the end goal of phasing it out in favor of a Latin Mass that follows the post-V2 rubrics. Francis was always disliked, if not outright hated by trads, so this recent move hasn't made him any more popular among them.