Why the app warns you about withdrawing everything
If you set a withdrawal to 100% of your balance, the app puts up a red warning and won't let you continue until you tick "I understand, withdraw everything anyway". It also offers Withdraw 75% instead. This page is why.
The pattern it's protecting you from
Your privacy in the pool comes from being one of many. A withdrawal is private because an observer can't tell which of the pool's deposits it came from.
Withdrawing your entire balance in one go weakens that in a specific way. If you deposited 1.2345 ETH and later withdrew 1.2345 ETH minus fees, those two numbers match each other and nothing else. An observer looking for pairs can connect the deposit to the withdrawal, and so your deposit address to the address you withdrew to. The more unusual the amount, the easier the match.
The warning fires on the whole-balance case because that's where the match is cleanest. A partial withdrawal breaks the pairing, since the amount leaving no longer equals any amount that went in.
What "Withdraw 75% instead" does
It resets the amount to three quarters of your balance in one tap. The rest stays shielded. You can withdraw it later, to the same address or another one, and the two withdrawals no longer add up to a figure that stands out.
Rounder amounts help further. 1 ETH out is one of many; 1.2345 ETH out is one of one. The same logic is why the app suggests round amounts when you deposit.
When it's fine to ignore
Sometimes you do want everything out, and privacy on this one deposit isn't the concern. Tick the box and continue. The warning is a speed bump, not a lock, and it's there so the linking is a choice you made instead of one you didn't notice.
Emptying an account with many deposits
A single withdrawal can draw on at most five deposits at once. If your balance is spread across more, one withdrawal won't take all of it, and the full-balance route that bundles many withdrawals into one transaction isn't switched on yet. Until it is, emptying a large account means several withdrawals, which also happens to spread the amounts out.