I hope this doesn't end up sounding like the Grinch who stole Christmas, but...
Unfortunately, broadband light-pollution filters MAY be nearing the end of their usefulness now that many cities are changing to LED street lamps. For example, both San Diego and LA are doing away with sodium vapor lamps within the next one to two years (completely, or so they say). Both of those cities are switching to broadband light sources which means that many of the so-called "light-pollution" filters may become less useful in the future (which is saying quite a bit since the effectiveness of broadband filters has been under some doubt even when sodium vapor lamps were the norm). Sad for astronomers, but this may be a fact of life in just a few more years.
Some of the cities are claiming that they will install better light shields to protect the night skies but I'll believe that when I see it.
However, narrow-band filters should continue to work, but those won't help much with galaxies, stars, and many reflection nebula (i.e. they are most effective when used on emission nebula and when recording narrow-band images using something like H-alpha).
In fact, I own a broadband Baader Neodymium filter and while it has a dramatic effect on sodium vapor light sources it has a much, much milder effect on LED lights (I've measured the change with a light meter and the difference is like night and day).
Frankly, however, I have yet to confirm whether the Baader Neodymium filter really makes that much of an improvement in my astrophotography. My images of the night sky no longer start out in a sickly yellow-orange but neither are they completely color neutral and light gradients are still a big problem. It's hard to do a direct comparison (with and without filter) since all of the images still need a fair amount of post processing to remove the broader effects of light pollution and to "stretch" the detail in the pictures. Thus, I can't be completely sure whether the differences (improvements?) are caused strictly by the filter or by the changes in the post processing (which is still kind of a black art for me and nothing close to a predictable science).
)
