In cases like this, being "handed a legacy system [with] no idea what the code does" is already a red flag. I've developed on plenty of legacy systems and the ones that went the best were the ones where I was given time to become familiar with the code first. Preferably with the going-out developer still around.

The fact that POs and stakeholders don't try to protect this knowledge exchange, and often see it as unproductive, is a major problem in our industry.