There’s two major reasons why deadlifting with a flat lower back is critical, especially if you’re an athlete
Reason number one is safety and durability. When you deadlift with a rounded lower back and put the spine into flexion, instead of the correct position of lordosis, you place immense amounts of stress on the lower back
Over time, that will manifest as irritation, pain, and even injury.
Now how fast you reach the threshold of injury depends on dosage and frequency, so if you’re weak and lifting light weights it will take longer to reach the threshold of injury than if you’re strong and handling heavy weights (sometimes on the spot).
Either way lower back pain and injury can be a massive negative not just for an athlete trying to reach their goals, but for living life
Everything in training should be to build up the athlete and make them MORE durable, not tear them down
Too many times to count I’ve been told of athletes having their goals waylaid and often derailed by being abused in their “weights” classes or “team workouts”, where the large amount of athletes and lack of coaching, and/or under qualified coaching, leads to disastrous consequences.
The deadlift is a great driver of performance, but it has to be coached, cued, progressed and regressed, and periodized correctly for EACH INDIVIDUAL
It comes down to not what you’re doing, but HOW you’re doing it