Even if I agreed that you have any right to do this (I don't), I just don't see how it works on any practical level.
Are we obligated to continue to acknowledge your character's history as well? Can we write them out of the game, as if they never were or do we need your permission for that as well? If the history up until the moment you leave the game must remain, how are we allowed to deal with the fact the character has just ceased to exist?
What happens if the PCs meet an NPC who interacted with your character (assuming the character hasn't been written out of the game)? What if things happened of screen with your character, that didn't matter then, but do matter now? Are we never allowed to discover what happened?
What happens if the character is replaced by a clone? Are you claiming ownership of the clone as well?
At a practical level, the character just rides off into the sunset. It's still out there, and could be scried or even chatted with, but anything more than that e.g. adventuring, killing it off, etc., can't happen without the player's OK. The history of what the character did while in play remains unchanged, including off-screen activities that may hold later relevance. (the clone question is a bridge I'll blow up if and when I ever get to it; the only cloned PC I can think of where the clone lasted longer than a single combat immediately following the cloning is one of mine-as-player, and that story's been going for 40 years now...)
In part this is to preserve the character(s) in case that player someday returns to the game, which happens now and then (and in fact happens for me, in theory, this Sunday).
In my current setting I've got loads of such characters.
What it means in practice is that if I-as-DM find I suddenly need an NPC Fighter for some reason then I'm going to roll up a new one (or use a pre-existing NPC) rather than recycle Falstaffe*, who was John*'s PC before he left the game.
And IME getting that permission is usually fairly easy if I'm still in touch with the player.
The one exception I'll make, as they can't affect the "real" character, are dream sequences.
* - names changed to protect the guilty.
If we want to, we can include (imaginary versions of) actual, real people in our game, without their permission. How do you conclude that you have more rights to the representation of an imaginary person that existed in a shared imaginary space, than real people do to their imaginary likenesses? What about characters from other sources of fiction -- should I get permission from an author before using their characters in my game?
In what way do you feel you are being negatively affected if the character is being used without your permission? What about if you never know your character is being used?
What I don't know won't hurt me; but if I were ever to find out, there'd be some harsh and unpleasant words said.
Is there any kind of time limit on your ownership, after which the character enters the public domain?
Well, I ain't likely to care after I'm dead.
Other than that, all you gotta do is ask permission - is that really so hard?
Edit: From a very practical perspective, looking at how this might actually work in reality, no discussion about the future of a character would be likely to come up before the player left, but the PC would not continue to play a major part in my game. Probably, some vague comment would be made about how the PC went off to rule his barony in his manor, and we'd continue. I'm not sure if you'd consider making that decision for the character a breach of your rights, but that would probably be the end of it. However, it may happen than five sessions later, one of the players says, "Oh, Lanefan's characters barony is not far from here -- we should drop in and see if he knows anything about what's going on in the area, and catch up for old time's sake." If that did happen, I would absolutely not pause the session to call you looking for permission. We would play out the scene, and I can't imagine any reasonable person having a problem with that -- although, most likely, you'd never know it happened, and would thus have no reason to care, anyway.
That's all fine and reasonable, and I too wouldn't have any issue with it. Where it would cross the line for me is if the DM allowed the party to successfully recruit my old character to go adventuring again, or were they to loot his castle while visiting, or kill him in his sleep, etc.
Also, I only DM for people I know, thus just because someone's left the game usually doesn't mean I've lost all contact with them. In your example here (but I'll flip the players around), if I-as-DM had any warning the PCs were about to visit your old character for an important conversation, during the week I might call you up, briefly explain the situation, and get some basic feedback from you as to what your character might do or say. And if during that call you told me something like "I don't care, do what you will with him" then you've given me permission to play the character as seems fitting, and that permission is all I'm asking for.
And a further edit: The version of your character that exists in your imagination is yours, and only yours. No one can do anything with it, other than you. The version of your character that exists in the imaginations of the other participants? That's at least as much theirs as it is yours, and you have no more authority over those versions than they do over what goes on in your imagination, especially if you are no longer participating in or engaging with the shared imaginary space were those versions exist. Any authority you ever had over the version of your character that exists in the imagination of the other participants, you only had because they elected to give you that authority (and if they were actually imagining your character in a way that's at odds with your own vision, even while you were participating, that's their right, as long as they weren't being disruptive about it).
Here I disagree. As it's my character, my imagination of that character (as best as I can express it during play) trumps everyone else's imagination of that character. They don't get to re-imagine my character in any major way just because I'm no longer there.
If you approach it from the perspective that the player might at any point return and take up playing that character again and that all you're doing in the meantime is in effect holding the charcter in escrow until that happens, my views might make more sense.
