for sake of learning, why is using slice not a good practice?

c = char = character
i = indx = index

how about naming? - c - it's not appropriate to call element like this

Agree!
How are those people who voted this solution with the clever mark??

Agreed!
Absolutely! no doubt!

I saw at least 10 solutions here - and all of them includes methods like splice or slice
and magic numbers like 3

yep!

all the solutions above - includes the number 3 - strange number.

Yes, not using v is ok. I could mark the first variable as "_" symbol to emphasize this point.

I know, but it's better to write it in one line using js modern methods, than use my own poor mistake decision

but why you don't use v anywhere?
is it correct for 'use strict' mode?

Overcomplicating the task doesn't mean it's complicated itself.