I suppose different funds would deal with it differently. As for IBIT, this is what I found on their 8K filing:

>> In the event of a fork, the Anchorage Custodian Agreement provides that the Additional Bitcoin Custodian may temporarily suspend services, and

may, in their sole discretion, determine whether or not to support (or cease supporting) either branch of the forked protocol entirely, provided that the

Additional Bitcoin Custodian shall use commercially reasonable efforts to avoid ceasing to support both branches of such forked protocol.

BlackRock also has shitcoin ETF, so I wouldn't trust them to do the right thing when the time comes.