So to close the loop on this, last week I flew JFK-HND-LHR in F both legs.

I always wore my mask when out of my seat and tried to when interacting with crew (as another poster noted, sometimes they surprise you).

I observed many F pax not wearing masks, even when moving about the cabin. The crew on both flights seemed not to mind, or at least never challenged anyone. And, all those passengers were Japanese. I was the only non-Japanese passenger on either flight.

Flights were great. Crews were lovely.

Only thing of note in terms of service differences was the lack of hot towels. But the pre-wrapped towels they provided (with the JAL First logo on the package) were really good for what they are. And the bathrooms still had cloth towels. Seeing as the crew picked-up and discarded the used pre-wrapped towels, I'm struggling to understand how they are safer than the old hot towels, but it's a minor issue.