Can Someone Explain using Luminance Filter in High Bortle?

No, that's the last thing you'd want to do. Use your Ha or some other combination of your narrow band frames to create your L.

You are confusing the image luminance (which all images have and which some people process separately; a luminance "layer" so to speak) with an L filter and its data (which is just a name for a UV/IR cut filter that passes all visible light). An L filter is often used to fill the luminance layer in broadband imaging, and gets the letter L for that reason, but is not meant to be used as the luminance data of just any image. It's needs to be appropriate to the use. The whole point of shooting SHO was to get the narrowband contrast. You shouldn't be looking to capture broadband data to throw all of that contrast away. Use your L filter for duty cycle stuff like focusing and plate solving during a session (with focus offsets when you change to your SHO filters).

I agree with the comments above.  I find that is best to keep the Lum filter "in my pocket" for any site that is  Bortle 3 or worse

Vinny Vent:
My question: Is there a benefit to shooting Luminance subs (with the Antlia or L-Quad) to incorporate into my NB images? To date on the few NB images I’ve processed I have been using Ha as a Lum per many of the PI processing tutorials I have watched. Would it be better to actually take real luminance subs?

Just to avoid potential for confusion, you can split this into two questions:

Question 1: Is there a benefit to incorporating luminance into narrow band images?
The answer to that is, in almost all cases, No. Narrow band maps atomic emissions to specific colors these different emission strengths are used to display structure and contrast. Luminance by definition is broadband across the visual range of wavelengths. The structural information gathered here will necessarily be different and trying to incorporate it somehow into a NB image will be, at a minimum, quite complicated.

Question 2: Is there a benefit to using something like the L-Quad from a light polluted site?
Potentially yes, but for RGB imaging, not narrow band. From what I can see, this filter blocks out some light pollution associated bands, so it is at least possible it helps you for RGB imaging the same way that traditional luminance filters would from a dark site. But you'd need to use this in conjunction with an RGB set, preferably one that excludes some light pollution bands.