Overclocking ATtiny1634

[PCB.Wiz]

Most MCUs have some sort of curves for MHz vs Vcc, and you can infer the temperature effects by comparing the same-die in differing temps specs.

Well, yes, but this basic inference is not what mean.
Let's say we take a sample of N=10,100,... MCUs and run all of them at certain voltage and frequency in temperature stabilized environment.
And we measure the time from power up to when a test firmware produces bad result or stops responding or otherwise diverges from other samples.
Then we plot mean time before failure with error bars against frequency at const voltage and const temperature t(f)|const V,T;
Then we also plot t(V)|const f,T; then t(T)|const f,V for all (f,V,T) points on a grid.
Eventually we end up with a 3D distribution <t>(f,V,T), and considering measured error also with <t>-e(f,V,T) and <t>+e(f,V,T).
It is interesting to see what this function looks like and what slices along coordinates and paths withing this (f,V,T) volume will look like.

That's a lot of measurements
You would expect classic bathtub curves, with zero failures inside operating region, and rapid failures outside

I did see this post, which seems to show one portion of a part being tested with MHz/Temp and Vcc curves so you can get an idea of Vcc and temp effects on limits
https://www.avrfreaks.net/comment/3231246#comment-3231246

[westfw]

You would expect classic bathtub curves, with zero failures inside operating region, and rapid failures outside

That sounds right.  With a desktop processor, overclocking might lead to over-temp conditions and decreased MTBF via heat damage to the CPU, but I think with a microcontroller, the issues are different, and you're likely to see it work "fine", up to the point where it doesn't work at all.