Can you find any documented instance where Liquid has prevented someone from pegging-out? Surely there must be at least one person who has been forbidden from pegging-out for this trope to carry on for last 5 years?

Because Liquid uses Confidential Transactions, bypassing any such "ban on your address from pegging out" would be simple to evade. Simply send your L-BTC to a few other addresses and peg-out from there. The federation wouldn't know who those addresses were nor what was being transferred....