We would not add something that could easily crash apps and then tell users to be careful. That’s not what we do. Our users expect us to guard against such things. At the very least we’d have to document what is and is not thread safe which would mean doing an extensive code audit.
The only way we would likely ever make this available would be by going through all the frameworks and making each and every method and property that is not thread safe raise an exception if accessed from a preemptive thread. That at least avoids crashing.
In nearly every case I can think of when a user’s app is crashing due to something in the Xojo language or framework, we consider that a very serious bug.