They exist on Upwork, but sometimes the clients act like they've hired a full-time, on-call, developer. You should have a call with anyone who may want hire you and set clear expectations before you accept any project.

Also when setting your rate, be mindful that they (like most platforms) take 20%.

Many years ago I used it when it was called ELance. At the time it was possible to find some short projects though it certainly took some effort.