I tested most of my engines with 2023.8 to make sure nothing had broken, I only found a single issue (a missing sprite_exists check in the open world 3D engine). array_length_1d is deprecated but trivial to replace (array_length does the exact same thing) so I figured there's no rush, there's currently no easy way to find a list of all deprecated functions in use in a project so I'd probably just miss something, think the project is safe and then run into a massive tech support nightmare when a new GM update drops anyway.
As for the for loop variable name, I'm just using "c" because there's a programming language called "C++" so writing "c++" as the increment operation is funny. That's the whole reason :P
Anything declared with "var" only exists in the piece of code it belongs to, so c's in different scripts will not affect each other. And with multiple loops in the same script, you reset the variable to 0 at the start of the for loop, so it doesn't matter it's reused. (The exception being when you have a loop inside a loop - then you need separate variables so the inner loop won't mess with the outer loop).