It comes and goes. For some time prior to Amazing Spider-Man #231 (published August 1982), it was completely missing. While he is sewing a new suit during #231, Peter Parker comments on why it disappeared for some time:
Of course, I'd have saved myself a lot of work, if I'd just silk-screened the design on the new bodysuits and let it go at that. But when I found a new supply of flex-nylon netting, I just couldn't resist putting the underarm webs back on my outfits. The netting on my old suit had ripped out long ago, and I couldn't find replacements for the longest time! I shouldn't have that problem now. This new brand is twice as tough as the old.
Yeah, that's the ticket! From straight on you hardly know it's there. But spread the arms and-- TA-DAH!-- Ol' Web-pits is back!
In-universe, it disappeared due to wear-and-tear.