I have a couple of questions regarding the CAP meta for gen VI. Though I'm not sure if theese are 'simple'.
I know the meta is not the Proyect's main focus and the current policy is to not update past CAPs. However Necturna is a bit tricky.
The first issue is that, in XY, Heart Scales can be used to relearn egg moves, so gen VI Necturna potentially has Smeargle's movepool, because it could relearn Sketch as many times as it wanted. This is in theory absolutely broken.
The second is sixth gen moves, disregarding multi sketch. Is it legit to use for example a Sticky Web Necturna? If the answer is yes then we have the best sticky webber, but we have inconsistencies with the policy (Necturna would then be "updated"). If the answer is no I feel the inconsistency is bigger.
For the first, as I understand it Necturna simply won't be allowed more than one Sketch move at a time, purely as a means of maintaining its concept; the fact that there isn't an in-game justification for such an arrangement is irrelevant, since as far as we are concerned at present, the CAPs are "locked in time" and Necturna perpetually retains the restrictions it possessed then, as a matter of principle. In this sense, Necturna can be subject to generational shifts (as you say, Sticky Web is one example, and I recall jas## was testing that very set on PS a while ago), but not actually subject to "updates".
In short, what we're doing at present is nothing more than pragmatism - we play with the CAPs in the CAP metagame because it's fun to play the CAP metagame, but too much effort to fix what would be continuity issues if applied elsewhere (e.g. Lightningrod Voodoom with its DP effect as opposed to BW, but the BW effect for everything else). Hence, Necturna has the XY version of Sketch, but we can ignore its repercussions in much the same way that in DP we ignored the fact that the presence of the CAPs would have affected certain egg move restrictions.
Hopefully, this should all become a bit clearer once we have got the CAP revisions PRC topic started.