This question comes up fairly often. OSHA classifies a scissor lift as scaffolding and therefore does require fall protection at ten feet and over. The guardrail around the work platform covers the requirement for fall protection unless it can be removed and if removed, it does require additional fall protection to be in place.
The confusion comes from some owners and general contractors that require the use of a harness and lanyard in addition to the guardrail system. Since they control the projects, they can enforce any additional safety requirements that they feel is necessary, but OSHA has no say in this and will not issue citations to those specifications, but only to their minimum requirements.
Where OSHA can start giving citations is if the manufacturers start requiring the use of harness and lanyards while in their equipment. OSHA will then be able to issue citations for not following the manufacturer’s requirements. At this time I know of no manufactures that require this.