This is a great collection of icons by Corey Ginnivan that’s both free and with no attribution required when you use them. The style is super simple. Each icon looks like older versions of the icons from macOS to me because they’re cute but not too cute.
Also? The icon picker UI is slick and looks something like this today:
Oh and also, as I was looking around Corey’s personal site I noticed this lovely UI effect when you scroll —each card stacks on top of each other: