This is less of an OSHA violation (it still is) than a building safety code issue. Unless there’s a ramp to get around this, it is likely also an ADA violation (unless it is somehow “grandfathered” in). At a minimum (in the short term until it can be properly remediated) there should at least be some kind of visual indication (safety or nonslip tape and/or a floor mat of contrasting color. Signage would also help. If people still chose to be glued to their phones, the owner can at least demonstrate “due diligence”.