Code executing in the JVM isn't sandboxed. Sandboxing could have indeed mitigated log4shell. Log4shell was a design where a too powerful embedded DSL was expose

Code executing in the JVM isn't sandboxed. Sandboxing could have indeed mitigated log4shell. Log4shell was a design where a too powerful embedded DSL was exposed to untrusted data in a daft way - the log("format here...", arg1, arg2) call would interpret DSL expressions in the args carrying logged data. One can even imagine it passing formal verification depending on the specification. But more broadly the thing is that eliminating these low level language footguns would allow people to focus on the logic and design errors.