In the exchange section it says:

Note that this is contingent on supermajority adoption by the Steem witnesses of the suggested upgrade, as is always the case with hardforks

Exchanges (assuming they are not also witnesses) should probably go ahead and upgrade to 0.21.0 regardless. If the hardfork is not approved then the upgraded code will continue to work with the existing consensus rules, and there should be no interruption of service either way.

The only way to know for sure if the hardfork will be approved would be to wait until the last minute and that isn't a good idea.

That said, some exchanges may explicitly want to disable their wallet during any hard fork just in case of problems. In that case it would be okay to wait and see.