The feet first slide has been a rule for years. It was a penalty. Worth it, but the right call.
Yes it has, but it used to be at the discretion of the referee, i.e. if the defender committed himself to the tackle before the slide then it wasn't a penalty, because there was no way to stop it.
With this hit, it was a late slide by Ryan, after Vaccaro had committed to the tackle. And now the rule is if a QB is sliding at all, no matter when he starts his slide in respect to the tackling defender, it is a penalty.
This a system primed for abuse. A QB can run 2 yards, see a defender coming, start his slide, and get an automatic first plus 15. It's an awful rule that goes too far into protecting QBs, the already get enough special treatment, they don't deserve it as a runner.
I OK with the slide, and giving yourself up as a QB, and if a defender begins to tackle after a slide, then personal foul. Fine. But with this, Vacarro was well committed and couldn't possibly adjust himself not to foul, because Ryan saw it, and did a late slide, abusing a system already geared towards offense.