There are two risks to consider with lenses, and both apply to pretty much every lens made.

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1. Risks from focused sunlight

Focusing sunlight readily creates a risk of heating materials to dangerous levels, which in turn creates the risk of fire.

Allowing sunlight to pass through an open camera lens is a fire risk, with many Lens & Body combinations being able to burn holes in the camera shutters. - Don't point your lens at the sun...

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2. Risk of damage from thermal expansion and overheating of parts.

This one really comes down to specifics of the design, and exact materials in question.

While it may be easy to assume that the risk is only with 'cheap plastic parts melting', it actually goes a bit farther than that.

Excessively cheap plastics might noticeably deform in temps as hot as you'll likely find in a car, but most of the risk comes from indirect damage and increase wear. The majority of plastics used for camera lenses aren't going to outright melt or directly deform from such temps, but they may be at risk of increased wear or damage if allowed to be heated too much before use. Plus the risk of premature end of life of a material if allowed to go through thermal cycling.

Thermal expansion is also high on the list of damage sources, and this is not remotely limited to Plastic parts. I have had an all glass and metal lens have its leaf shutter buckle due to heat, and I had used a piece of optical equipment as part of a scientific instrument package that would pull itself out of alignment if we let it get over 40C.

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Finally there is also another risk: Excessive UV damage to the material if left in sunlight. However this is probably unlikely for the majority of lens models unless left in direct sunlight for ages.