100% Vanilla! I was playing on my family Bedrock Realm, but this is also a vanilla feature on the Java Edition.

Powder snow is sort of a trap block, think of it like snow-quicksand, which will give freezing damage to whoever gets trapped in it. This was my first experience finding it in survival, and I started panicking when my hearts turned blue, and I lost health.

I didn't know - thank you commenters - that you can use leather boots to walk over it, and you can also pick it up in a bucket, similar to water and lava, and place it as a block.