It's only complicated for the wealthy. The majority of taxpayers can fill out their tax return in about an hour at most.
All income is taxable, always has been. If people really had no idea, that's on them. I'm no fan of paying taxes like anyone else, but I try to know the rules. It's not that hard to keep up.
Venmo in itself isn't taxable, but if you earn income using Venmo, that income is taxable. Venmo will need to report, just like any other business does.