What about this idea to help prevent the manipulation of ratings?

"Is it a good idea to restrict a players game seek to a maximum of 200 points below their rating in rated games? No upper limit."

No, this is not a good idea. It's basic statistics. If a rule like this were in effect, it could result in potential competitors forcing each other out of a competitive environment. (Instead of ratings manipulation by dumping games or cheating; it would be ratings manipulation via specific system rules meant to undermine the system and the rules. It would designate a different set of domain rules that would then skew a game in favor of a few select players.)

Simply put: If I'm higher rated than you, then all I need to do to ensure I never compete against you again is lose or draw to one of your competitors in a field within a certain range such that I don't lose enough points to have to compete with you. In a manipulated system, even with a draw instead of a loss, you can play the margins to capture and manipulate the competitive field of play by proxy.

Variations on a theme. Changing the rules of the system (overall) is tantamount to, or no different than any other ratings manipulation. (This cannot be argued unless you can show it from a statistical standpoint that the existing system is worse than a better system.)

Made not an ounce of sense.
For what reason would would a 2000+ player want to play a player 500 points or lower a rated game? Casual games, friends, coaching etc, etc, sure.
1500 Players can always set their seek to a maximum, say 3000 and any such player can accept a challenge from lower rated players.
The restriction applies to players from setting their seeks very low. When posting a challenge , they'd be prevented from seeking games way below their rating. There is absolutely no reason to be playing rated games vs. players 500-1000 points lower, unless it is to game the system, to gain points or throw points, to manipulate either players rating.

The suggestion is not meant to "solve" the problem. But to "help prevent" rating manipulation by making it more difficult. Players that refuse to accept challanges because the opponent is to highly rated can be more readily sanctioned. As Toadofsky pointed out, there will always be ways to game a rating system. The objective is to make it as difficult as possible, and have rules in place to enforce those who try to circumvent pairings.
To be fair 300-400 points is a more realistic number that shouldn't be objectionable.

Your idea is stupid and I will bet my Trophy that it will never be implemented. Playing low rated players is not manipulation of any kind. Why do you think IRL players would rather play up than down their rating? Because they have much more to gain than lose.

If a 2000 player plays a 1500 and wins he will get 3 or 4 points. If he loses he goes minus 50. Now if you put a 1500 player against a 2000 and make them play 10 times the 1500 only needs to win 1 game to get the same amount of points as the 2000 if he won 9. Draws will decrease the better players rating as well.
This is how the rating system works universally - it balances itself out.

They'll probably just get rid of the 0+1 time control so I wouldn't get too excited MDinnerspace.
Infact here is an idea. For one week play only 1500 or under rated players yourself and you will soon realize how silly you sound. Good luck.

@NeverBeenTimid

It is obvious to all you are gaming the system by only playing correspondence. Of your 100 RK games 90+ are against 1500? players who have never played a single game of Racing Kings. They are all brand new sign-ups to lichess. You are not creating any challenges in the lobby, seeking games vs only 1500? players. These new players are not posting RK challenges for you to accept. Many of the players have barely played a single blitz game when they find themselves in a RK correspondence game with you. If you were using the pairing system, you would have played games vs all ratings, just as you play all ratings in all your other types of games.
Of those 90 games, the vast majority of players (1500?) make 1 or 2 moves and time out. They soon realize they are in one of the craziest variants without a clue to the rules.
You track new sign ups and send a message challenging them to a game they have never heard of nor seen. They quit straight off, you get the points. These are the facts. Your reasons for choosing this method to find games are your own. There is nothing in the rules to prohibit it. By definition, it is gaming the system, playing only 1 specific rating group of players.
You clearly are opposed to my suggestion, as it would restrict yours and others from gaming the system in several ways besides your own. It can only be called "rating manipulation" when, by example in your case, of those 100 games, 80+ games never go beyond move 3, as the new sign ups only accepted a challenge sent to them via messaging and not by the usual route of using the pairing system.

My concern, reason for the suggestion is not over this trivial issue explained in the post above. The main issue to my eye is new sign ups, starting with 1500? can play 2000+ rated players and very quickly, after a few wins, they reach close to a 2000 rating.
Glicko 2 awards far too many points to provisional players. The opportunity is always present to play "friends" with established ratings who are willing to lose about 20 points, enabling a new sign up to gain close to 200 points.
How many of you set your seeks 500 points below your rating? Very few I'll wager. Players seek competitive matches, not much point in playing games vs players rated 500 lower is there?
Of course this applies to rated games only. There are many good reasons in casual play, exhibitions, coaching, simils etc.