When LED Bulb packaging says NOT FOR USE IN TOTALLY ENCLOSED FIXTURES is this stating a Safety issue or that it may affect the Lifespan of the Bulb itself?
I believe that is a copy over from the CFL days and UL 1993 " Self-Ballasted Lamps and Lamp
Adapters" which they borrowed from when they made the standards for LED lamps. the standards have evolved over the years.
My understanding is at UL they require a sample of the bulb to be tested in many different fixtures with their respective lens's, if a fixture can have more than one style it can be very expensive to have each bulb tested in each lens style UL has for a standard A19 size.
So manufacturers opt to test the bulb in a A19 fixture with no lens
and UL stipulates "There shall be a marking to indicate "Not for use in totally enclosed recessed fixtures," unless the device is tested with the lens "
