Many people believe it, including several people I know. They say it's to cater to kids who think they're cats because they have slippery slope brain and think accepting trans people means we're also starting to consider otherkin types as normal. It's like the old homophobic schtick about how allowing gay marriage will lead to people marrying their dogs.

As someone else pointed out, the only grain of truth is that classrooms are keeping kitty litter on hand, but it's for if kids get confined to a classroom for an extended period of time due to a mass shooting.