Bitcoin difficulty re targeting in case of catastrophic hashrate drop?
But how is difficulty changes if suddenly 90% of hash rate disappears* ?


It will change as it normally does, after 2016 blocks. There is no provision for adjusting the difficulty if the hashrate suddenly drops.


  *recent BCH fork had difficulty adjusted quicker than 2016 blocks.


As part of their hard fork, they added a provision for cases where blocks are taking too long (i.e. sudden hashrate drop). This extra difficulty changing rule is not present in Bitcoin.