Parmandur
Book-Friend, he/him
Obviously I haven't seen Theros yet, but I hope you're right, and it's a strongly-worded limitation (which obviously any individual DM can rule 0), because the "limitation" @Urriak Uruk quoted from Ravnica is not a limitation at all, but rather a mildly-worded suggestion that some races might be more or less suitable (and doesn't appear to limit classes/subclasses at all).
As people are saying, Dark Sun needs something vastly more severe, something that clearly lays out that by default, no non-Athasian races should be present, no normal Cleric subclasses should be present, certain spells should be absent, and so on. As I said, in a later DM-oriented chapter they can have a paragraph on flexing this, but the entire setting should be designed on the basis that it holds true.
100% agree. They're mantis-beings, not humans with bug-ish features and maybe extra arms. That DiTerlizzi one is really good. They need to have an abdomen and a totally non-human physique. A lot of the more recent stuff for them just makes them look like 4-armed, vaguely insect-headed humanoids who have a humanoid torso (rather than a thorax and abdomen separately like an insect), and don't even have a real exoskeleton, just bony plates on top of relatively normal flesh.
Given Theros is adding Centaurs, who are far more extreme, this surely cannot be a real problem to do either.
The difference between that level of restrictions and what's in Ravnica is of degree, rather than kind. EotC has shown that restrictions for a Setting are a thing that they will do.