When a stock fails to meet the requirements of the regulated exchange on which it's being traded or does not wish to comply with such, more often than not, it gets moved to or begins trading on an OTC exchange (Over-the-counter).
In its nature, Over-the-counter exchanges differ substantially from the regulated venues such as NYSE, NASDAQ, etc., which is why they do have certain restrictions and limitations in terms of who can access the stocks offered through them and hold ownership on.
You can find more information about the OTC venues available here: OTCMarkets
Some stocks traded on the OTC market are not permitted to be held in an ISA. We have an obligation to remove any such holdings from ISAs once they are delisted from the regulated market to OTC. You can, however, sell the securities you are holding but we cannot provide market data beforehand. If you hold such stock in an ISA and have not sold it yourself, we will liquidate it within the period of a working week. The proceeds will stay in your ISA.