No-one would know that you+BankCo@example.com is an email address you can even be reached on, unless it leaks out from BankCo.
ADD: and of course, you aren't really going to stick +BankCo after your real name to generate an email to use with BankCo. You're going to give them something generated like you'd generate a password - "ol48eILm@example.com" or similar - so if anyone finds out that particular email address for you it doesn't tell them with whom you use it.
ADD: and of course, you aren't really going to stick +BankCo after your real name to generate an email to use with BankCo. You're going to give them something generated like you'd generate a password - "ol48eILm@example.com" or similar - so if anyone finds out that particular email address for you it doesn't tell them with whom you use it.