Right off the bat, an aesthetic is not the same as part usage. Some parts will lend themselves to an aesthetic more or less, but using a piece will not immediately break with an aesthetic. So while it’s fair to say that all 2.0+ Hero Factory sets had a different aesthetic to Bionicle G1, and that CCBS generally lends itself less to achieving that aesthetic, I do not believe that can be extrapolated to say that any usage of CCBS (or even an entirely CCBS MoC) can not be stylistically in line with G1.
So while you can justify “I avoided CCBS to make it feel more G1”, I don’t think it’s fair to say “that MoC has CCBS, therefore it cannot be G1.” You could say that the way it has been used breaks the style, but to say that the mere existence of a CCBS piece invalidates a build aesthetically is the wrong argument. That is trying to argue that CCBS should not belong in Bionicle canon at all, and while it’s a separate discussion, it’s got nothing to do with aesthetics.
All that out of the way, I don’t believe a hardline, qualitative “G1 aesthetic” even exists. However, there are various things that such a term may refer to when people use it:
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Some people who say “G1 aesthetic” really mean “01 aesthetic”. They like the technic heavy builds of the Rahi, and the style of the sets from the “Golden Years”. While this is a much more specific style (characterized by an emphasis on the biomechanical, generally gappy builds, and other effects most easily replicated with technic parts) it should not be confused with “G1 aesthetic”. The Glatorian were from G1, therefore, they ought to belong in any hypothetical G1 aesthetic too.
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Sometimes it means part usage from 2010 and prior, in an attempt to avoid using “anachronistic” pieces in G1 builds. This, as I explained above, is not an aesthetic at all - it’s a parts usage debate. Any part can be used to create any visual style, if used cleverly enough.
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Sometimes “G1 aesthetic” just means “not G2 aesthetic”. G2 arguably had a more consistent and definable aesthetic than G1 (smoother builds, more recognisably humanoid shapes, bright colours, and blocky, filled-out silhouettes). G2 sets do look quite distinct from G1 sets - after seeing a few, most non-Bionicle fans could easily pick one out from the other - but the fact that G2 looks out-of-place in G1 does not make G1 more specific.
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99% of the time, G1 aesthetic means “G1 set aesthetic”. This means making the build match the visual style of the G1 sets without necessarily being strict on part usage. While these sets all have some things in common (for example, Nick Bluetooth would not fit this aesthetic) it’s really diverse and so it’s somewhat difficult to quantify.
Almost universally this term is used in reference to sets, something that annoys me personally. The Miramax films, “Bionicle: The Game”, TLR, Stuart Sayger’s comic art, and characters described in serials all are a part of G1. I get a bit cheesed off when people bash a MoC for “not looking like a G1 set” when it clearly imitates one or more of the things we’ve seen outside of the sets. Fully organic creatures, eldritch blobs of tentacled gelatin, completely invisible beings, entirely inorganic robots, pure elemental entities, and characters with hair all exist within the world of G1, which is why I say that a G1 aesthetic doesn’t exist at all.
I believe it’s more important for a build to fit the aesthetic of where it belongs in the G1 universe - something that looks overly organic or muscle-like would make a terrible Marendar but a great Gold-Skinned Being. Something made entirely with translucent parts and spikes would not at all be appropriate for a Toa of Lightning, but be a superb Avohkah. A humanoid made exclusively with pre-06 pieces would make a perfectly fine Toa, but would be very hard to make resemble a Skakdi. And someone like Annona will almost never look correct in a set form at all - yet she is just as much a part of G1 as anyone else.
It all depends on context, so I don’t think saying “this doesn’t look like G1 aesthetic” is a very strong argument because G1 is so styllistically diverse and cross-genre. The only stuff that really looks out of place, to me, is something with a very strong resemblance to real-world concepts that wouldn’t exist in Bionicle. Beyond that, most things can work if they are used in the right place.