FWIW, on Tuesday, I booked PHX-LHR using a SWU for a January, 2024 departure. I was hoping SWU availability would open up on that flight and I have been checking that route for the last couple of months. During that time, I saw a variety of SWU availability from ORD, JFK, CLT and DFW to LHR in January, 2024. So, I believe SWU availability is indeed available for at least a few long-haul routes 4 to 5 months out during the off-season. As departure time approaches, another window may open up if there are still seats available.

An interesting side-note. I paid just under $1,000 for the PHX-LHR flight on Tuesday. On Thursday (yesterday), I noticed the fare drop to around $700 but no swu availability. Normally, I would cancel and re-book but that would not work as there weren't any swu availability and was unsure if I canceled that swu would then become available. Called EXP line and they were able to re-use my SWU on the same flight and gave me an approx. $300 credit. I thought that was a very nice gesture and proves it never hurts to ask.