I think:
function missingOrNot(data::Union{Matrix{Float64}, Matrix{Union{Float64, Missing}}})
or
function missingOrNot(data::Matrix{T}) where T Union{Float64, Union{Float64, Missing}}
I think:
function missingOrNot(data::Union{Matrix{Float64}, Matrix{Union{Float64, Missing}}})
or
function missingOrNot(data::Matrix{T}) where T Union{Float64, Union{Float64, Missing}}