Why couldn't they just say "Not Tested for use in Totally Enclosed Fixtures" or similar?
I have no idea, what UL is thinking, that warning in my opinion leads to a lot of confusion and is probably meaningless.
I think as long as the input watts of a LED lamp are equal to or less than the watts on the enclosed fixture its a non issue.
We're used to seeing lamps (aka bulbs) for sale in North America typically have a mark from a Nationally Recognized Testing Laboratory (NRTL) indicating compliance with the safety regulations outlined in a standard.
While we have always accepted UL standards such as UL 1993 and UL 8750 for LED lamps (that require that testing) I am not aware of any law stating the UL standards *must* be used, there probably is one I just don't know where it is.
The International Electrotechnical Commission (IEC) develops standards that are used everywhere else and here, and they have standard 62560 specific to LED lighting.
IEC 62560 appears to be very similar to UL 1993 and UL 8750 as they both detail mandatory markings, construction safeguards, minimum resistance to hazard sources and operation under fault conditions.
According to the IEC 62560 mandatory markings include a mark of origin such as a trademark or manufacturer/distributor name, rated voltage or voltage range, rated wattage and rated frequency. LED lights not suitable for dimmer circuits are required to indicate this clearly in installation instructions or package labeling, however I don't think it has that verbiage about enclosed fixtures I could be wrong as I have not read both.
At one time the voltage a bulb ran at was very important and so a North American (UL) Standard was necessary, but in 2025 its questionable if we even need UL 8750 if the same thing is covered in IEC 62560
https://webstore.iec.ch/en/publication/7259http://www.ledbenchmark.com/faq/LED-standards.html