Create a new consensus method that ignores guardfraction votes
And here is how we remove code that depends on consensus method 20 (the guardfraction method):
Create a new consensus method that ignores guardfraction votes. This new method can go in 034.
Change all the code that says ">= MIN METHOD FOR GUARDFRACTION" to say ">= MIN METHOD FOR GUARDFRACTION and < MIN METHOD TO IGNORE GUARDFRACTION" (the new method). This disables guardfraction when the new method or any later method is used. See consensus method 28 for an example of the code and spec that we need to remove a consensus feature.
When we will never revert to a lower consensus method (two releases later, 036), stop authorities voting for the buggy methods 20-28, and remove a whole bunch of old code, including all the guardfraction code.
At that time, we might want to remove all the methods lower than 20, too. See #24378 (moved).