I don't think Riot has betrayed the lore of the Vastaya with Sett's design.

The fact is that when the Vastaya dropped as an actual race, the baseline we were given was Ahri. Following her we were given Xayah and Rakan.

The 3 of them were the race's mascots. So I don't know why people are acting like designs like Neeko or Sett are 'betrayals to the lore' for not being more monstrous. It was pretty clear from the beginning that Riot wanted Ahri style mixtures to be typical of Vastaya, and more bestial Vastaya like Rengar to be uncommon, or even rare.

Of the given Vastaya, he's the only one who's that bestial in nature.

The other Vastaya have a few animalistic features.
Ahri has whiskers, ears, eyes, and tails of a fox. Xayah and Rakan have feathered ears and the lower legs of birds. Riot is even vague on whether their wings are actual wings or just cloaks they wear. Wukong is literally just furry and has a tail. Nami is scaled and tailed, with a few fins.

In contrast, Rengar is the only playable Vastaya who is as bestial as he is.

And this makes sense for his character and his environment. The vastaya, all of them, can modify their shape.

Ahri wants to fit in, so of course she'd be more human like.
Xayah and Rakan both grew up in tribes with high contact with humans and diminishing magic, so they look as human as they do because they're defaulting without their magic. Neeko has seemingly only recently adopted her current form, trying to adapt to these new people while still keeping her favorite parts.
Wukong is as bipedal as he is so he can more easily practice the Wuju style, which is designed for humans. Nami is almost entirely animalistic barring a few things humans would've found attractive.

Rengar? He was in the Kumungu jungle, which absolutely flourishes with magic, and he had next to no contact with humans while having a desire to be as fierce as possible. His form makes sense for his personality and his environment.

Sett? He grew up in a magic starved fight pit, while longing to fit in with those around him. Of course he's just gonna have ears. Conspicuous enough he can't hide them, but still mostly human. He doesn't have the magic for a more bestial form, nor the desire for one. And that all compounds on his mixed ancestry.

Yes there are more animalistic Vastaya, but they aren't the typical Vastaya. And Riot never intended them to be.

Riot has blatantly always intended Vastaya to be animalistic people, not anthropomorphic animals.

Neeko and Sett aren't betrayals. They're logical conclusions.