IBC-2015 2406.1 (Exception)

I would say yes, a solid wall or other solid substrates that would prevent a person from walking completely through the glass to the other side.

 

2406.1 Human impact loads. Individual glazed areas,
including glass mirrors, in hazardous locations as defined in
Section 2406.4 shall comply with Sections 2406.1.1 through
2406.1.4.
Exception: Mirrors and other glass panels mounted or
hung on a surface that provides a continuous backing support.

I would accept a solid surface backing that won't bend. Careful, most new medicine cabinets mirrors do not have a backing.

 

True, but medicine cabinets typically do not fall into one or more of the "hazardous locations as defined in Section 2406.4..."

The intent here is for locations like in a weight room or dance studio. Mirrors are typically installed with a minimum 1/8-inch gap behind them to allow for air circulation to prevent the accumulation of moisture behind the mirror that could deteriorate the silver coating. So some slight bending could occur. What you want to do is make sure the bending is not enough to cause breakage when impacted by a person that would allow the person to be temporarily within the opening where falling glass could cause injury. However, no level of backing will prevent breakage when impacted by a heavy concentrated load, such as a barbell.

Whether required or not, I would always use safety-backed mirrors in these types of locations.