Nobody slides feet first on steals anymore. Theory is headfirst gets you there faster; presents less surface to tag, and you can use your arms to avoid tags (e.g., swim moves). Since everything is measured and analyzed these days, I’m going to assume these are true.
Now, I think if you have an infielder who tends to block the base with his knee or feet, a feet first slide will break said infielder of that habit quickly.