As the error says, "formulas that reference this field are no longer valid." Formulas that reference other formulas end up being the size of the dependent formulas plus their own size every time they are referenced, which can cause this problem.
For example, let's say that your formula was simply:
Then you write a second formula, such as:
IF(Created_Date_Example__c < DATE(1999,1,1),
"Earlier than 1999",
IF(Created_Date_Example__c < DATE(2000, 1, 1)
"Earlier than 2000",
"Later than 2000"
Then, the second formula uses the computed size of the first formula twice. Obviously, this would still compile, but if you later made
Created_Date_Example__c more complicated:
IF(OR(CreatedDate=NULL, CreatedDate<DATE(2000,1,1)), DATE(2000,1,1), CreatedDate)
Then the second formula's size would increase as well, even though you're not editing it. Obviously, this example still compiles, but it's rather easy to go over the maximum size in a dependent formula if you do this.
You'll need to try and optimize your other formulas, or you won't be able to make any further changes to this one, unless the result is smaller than before. I doubt you can optimize this formula to a smaller size, so you'll need to review the other ones.
Alternatively, you might create a new numeric field, and use a Workflow Field Update to set the value for this field. This eliminates the need for this formula field, which will reduce the size of the formulas that depend on it.
Also see Knowledge Article 000324937 that goes into more detail, and Tips for Reducing Formula Size.
Summary: this formula is used in other formulas, so their size is going over 5k characters compiled. You need to fix those formulas, instead.