Preventing forks from breaking away with a shared tx history prior to some block is impossible. If you're going to hardfork away from decred, then you can do anything you want to maintain the tx history. For instance, at block X empty the ticket pool, reset ticket price to 1 dcr, reset PoW diff and change algo, if necessary modify other checks to allow ticket pool to be emptied, etc. It's ludicrous to pretend that no one could hard fork decred and start a split chain with a shared tx history prior to some block. Would it be more of a pita than doing the same thing to Bitcoin? Sure. Would it be impossible? Controller nuclear fusion isn't impossible, this sure as shit isn't impossible.
I found the article!
What would happen to Decred in a Bitcoin Cash or Segwit2X hard fork scenario?
Firstly, Bitcoins hard fork problem would simply not happen in Decred. There is a formal on-chain voting system to deal with all governance decisions related to hard forks. Secondly, it would be virtually impossible to fork the Decred network in the way we have recently seen with the Bitcoin Cash fork or SegWit2x. This is because all new blocks created by PoW miners need to conform to the wishes of Decred stakeholders (PoS miners). If PoW miners refuse to comply with the collective policy, PoS miners can invalidate any new blocks they create. So, for example, if 80% of stakeholders voted in favour of activating the Lightning Network, and the remaining 20% refused to move over to the new chain that supported Lightning Network features, then the 80% majority could simply vote new blocks on the old chain as invalid, which would strip them of the block reward. In turn, this would incentivise PoW miners to move from the old chain to the new chain, and the old chain would die off.
The only way someone could hard fork Decred against the wishes of stakeholders would be to create an entirely new fork of the blockchain and start it from scratch. In which case, it would not be contentious at all, because it wouldn’t be Decred.
Source:
https://thedecreddigest.com/2017/11/08/hard-forks-done-right-its-decreds-time-to-shine/ (just before the conclusion at the end of the doc).
TL;DR: a chain split where the original history is shared in the contentious hard fork is
"virtually impossible" in Decred, because voters are incentivized to vote against "contentious" PoW blocks being created. Without PoS votes confirming the validity of the block, those blocks offer no block reward for the miner, incentivizing that minority to move from their unprofitable and contentious fork to the new chain.
) and they're more focused on the bigger picture than any short-term results. I think that's the right philosophy to build a really solid project. 
you new here? 