The witness vote decay rule was introduced in HF25 and results in the witness votes expiring after 1 year of no witness votes being made by the account. I'm in the process of adding a script to display what this is going to do to the witness ranks when it goes live - to the new Hive stats site I am building.

I think the format of this new rule seems reasonable, though I imagine that some people will just run a bot to make a new witness vote every so often automatically.