ARK has a very experienced board of trustees for the ARK ETF Trust, which oversees the ETFs with a wealth of knowledge, insights, and expertise.
In the case that an ETF closes down, it has to follow a strict and orderly liquidation procedure. Shareholders typically receive notification of the liquidation between a week and a month before it occurs, depending on the circumstances. The board of trustees of the ETF, will approve and oversee such procedures.
Investors who want "out" of the fund upon notice of the liquidation sell their shares; The remaining shareholders would receive their money, most likely in the form of a check, for whatever amount was held in the ETF. The amount of the liquidation distribution is based on the net asset value (NAV) of the ETF.
Disclaimer: Please note the answer above may be incomplete and offers only a limited explanation. Investors should consult a tax professional or financial advisors for more information regarding ETF liquidation procedures.