"Doesn't exist? I'm confused:"

My tongue is firmly in cheek. If it's metal but not lugged it doesn't exist. If it's aluminum, it could be lugged and still not exist.

"Steepen the head angle by 1 degree and raise the bottom bracket by 5 mm? Fairly minor changes. Do we need a seperate frame category for this kind of frame?"

"Oh reason not the need", to quote King Lear. Sorta like why we need all those 700C cyclo cross bikes, and couldn't live without 29ers, or 69ers, or...

It's fair to ask if the bike ends up within the parameters of another bike is it a different bike. It would in part depend on what the intention of the designer is. I find it hard to believe, that if you put the objective of crit bike down on a piece of paper, there wouldn't end up being a whole series of minor optimizations that might actually be tried. In tubing, design, layout of some parts, and so forth. Just as any two designers might not come to the exact same formula when preparing a given design for anything. If there can be quite different approaches to the same task. If indeed they take 30 years of professional slogging to get right. It is hard to believe that with a modified objective nothing new could be added or subtracted. Now whether there is any real difference, who knows, but that's the kind of thinking where you may as well turn out the lights. The business of custom bikes is not the business of zero distinctions.

I do wonder whether the reason micro differences like this aren't there any more is that it is more expensive to do them with some processes like all carbon molded frames, or whatever people win with at the highest level. Or perhaps there isn't an advantage to a world class athlete who like Lance, went through a whole development program tailored to his needs, to do that all over again for a "minute change". Maybe the hyper-perfected bike is a better deal than spending the time on several less well optimized frames. On the other hand. In the steel world many of us occupy, in the weekend warrior class, were some folks don't have a whole development program for each bike. Maybe it makes sense to get a bike built to the only style of race you actually participate in. Steel can make these adjustments easily. Maybe we (or really, all you all) have an interested in hanging onto minute differences.