It does, see https://www.nushell.sh/book/custom_commands.html
def greet [name] {
$"hello ($name)"
}
They were added in 0.25 in January 2021 (https://www.nushell.sh/blog/2021-01-05-nushell_0_25.html#cus...)
Last time I checked it didn't have functions. Has that changed?