The easily readable structure of having an if keyword which controls execution of a following block of code is so prevalent in programming that it seems to just be part of it.

However, programming started out with machine code that was far away from the easy to understand

if condition
    do thing

What was the first time a programmer was able to use this simple and intuitive way of executing code based on a condition?

This excludes IF...GOTO statements, as that - in the sense of this question - only allows to execute a GOTO statement, opposed to arbitrary code. Also condition and code to execute are split up there and not together anymore. This is more about the "usability for programmers" perspective than about the technical ability to support this feature.

As user Raffzahn mentioned, this can be called "the first (block) structured language".