Why did you add declares then ??
Declares is the easiest way you can find to crash app. Wreckless user will crash it instantly there.
You should not make Xojo cheap so people have to seek other tools just because wreckless user could crash it. While other users need it to be able to grow with the tool.
And going through the framework adding exception if method is not “thread safe” is not going to work either since that is just not how things work with threading.
Non thread safe you can use in thread. Just cannot use same instance of the given object in more than one thread at once. For example if Date was not thread safe then I still could use dates in threads…two threads just could not write to same date object without setting up mutex handling to guard for it.
There is of course other kind of not thread safe, that is user interface. But that is different story, and really just same story as with current Xojo threads.