Are rear triangles made today made of thinner / less substantial materials than in the past?
There is some validity to the observation.
In aluminum, hydroforming on mainstream bikes only started in the early to mid aughts and got more prevalent and sophisticated from there, eventually to the point where even fairly low end bikes often have heavily manipulated tube shapes that would have been much more expensive to produce, if feasible at all, in the past. Hydroforming is often used now to create thin profiles on chain and seat stays that in the past on a comparable price point bike would have been a chunkier, continuous diameter tube.
In carbon, many companies have veered towards dainty, compliant rear end sections. There was less of this in the earlier days of carbon.
In steel however, there's been some of the opposite effect. Steel stays on many contemporary bikes are much larger diameter than comparable older bikes. In some cases this is because fancier modern alloys beget larger tubes with thinner walls. More commonly though, it's a mix of consumers being visually accustomed to bigger tubes and also the structural requirements of disc brakes.