This is the issue for being able to change the module into which the REPL evaluates: https://github.com/JuliaLang/julia/issues/22562. There have been some atttempts to implement this:
Once it’s possible to attach a new module to the REPL, then it should be easy to create a new Main module and attach that to the REPL, which should have the effect of clearing the workspace. What’s needed is for someone to roll up their sleeves and finish implementing this.