That honestly feels like a random, implicit thing a very shallow-thought-through esolang would do …

Nope, you're far from the truth there. Most functional programming languages have this feature, but it's also definitely not shallowly-thought-through, as it's essentially an extension of how maths works.

Basically, in most cases when you see braces { } (excluding things like for-loops and imports), you can think of them as an expression, where the whole brace-scope will evaluate to just one value, similar to how "3+5" evaluates to a value. That one value is this last value at the end of the brace-scope.

So, to give a very simple example:
{ 3 + 5 } / 4 evaluates to
{ 8 } / 4, so then the whole brace scope evaluates, which gives us
8 / 4 and that's then
2.

In maths notation, you know that as (3+5)/4, with parentheses instead of braces.
Within this simple example, they do the exact same thing (and Rust does also allow you to use parentheses for this purpose).

Where braces and parentheses differ, is that braces allow you to write multiple statements within them, so in theory, you could do:

{
    let x = 3;
    x + 5
} / 4

Obviously, this is where this simple maths example largely stops making sense, but in real-world programming, there's a lot of use-cases for this.

It does take some getting-used-to, when you're coming from hardcore procedural languages like C/C++, but yeah, it's really not new for anyone who knows maths.