It comes down to the word "enforce".

It's general practice on CX (although not 100%, as Carfield's post illustrates) for FAs to request pax lower shades on longhaul flights. (I'm speaking to J and sometimes F, not as familiar with Y). This includes "daytime" longhaul flights, like the daytime flights from North America to HK or HK to Europe (only London for now; used to have Paris pre-Covid). Or, if you're not in your seat or sleeping, they'll proactively lower it for you.

However, you are fine to leave open. Some FAs will push harder than others but you can keep open. You can say "I prefer to keep open" or something along those lines. You might get some pushback, but ultimately you'll be allowed. It's not like the police or some air safety rule. But if you can't handle a little debate, then you may end up with them closed.

I lack appetite for the debate over what's proper etiquette. This is just to say, if you wish you can keep them open. IME some pax - I think of Western countries where I'm from - aren't used to doing something different from the FA request, or are more easily offended by different things. Chalk it up to cultural differences about sensitivities (I think of this ongoing thread). So sometimes hurt feelings and/or bad experiences can result from cultural or communication differences.