That's not true at all. If your shares are held at a brokerage firm, that firm will "re-org" your shares directly into your account.
If you bought shares in the open market, you can NEVER legally be blocked from selling them.
You and your broker just need to be aware a split happened and make sure you adjust for the split. The re-org could take a couple of days, or it might happen concurrently with the split. It will depend on your broker and DTC.
But if you bought stock in the open market, you can never be blocked form selling them as long as the stock is trading.