By virtue of "The House Without a Key", Charlie's first appearance, having been published in 1925, the intellectual property is now in the public domain, for any and all to use as they so desire.
One caveat, only those aspects of Charlie's character that are extant in "The House Without a Key" are free to use. Any additional bits added to the character through the various other adaptations over the century are still under copyright to those entities that created them.
Yet, as each year rolls by, more and more of the character's literary background will also fall into public domain.
However, the main take away from all of this is that like Sherlock Holmes, Charlie Chan is now free for the masses. Here's to hoping that 2021 brings him the sort of fame that that other detective has known!
