This has never stuck with me, and I hadn’t thought about why until now. I have two reasons why I will always write ${x}_$y.z instead of ${x}_${y}.z:
Syntax highlighting and shellcheck have always caught the cases I need to add braces to prevent $x_ being expanded as ${x_}.
I write a lot of Zsh. In Zsh, braces are optional in way more cases. "$#array[3]" actually prints the length of the third item in array, rather than (Bash:) the number of positional parameters, then the string 'array[3]'.
This has never stuck with me, and I hadn’t thought about why until now. I have two reasons why I will always write
${x}_$y.zinstead of${x}_${y}.z:$x_being expanded as${x_}."$#array[3]"actually prints the length of the third item inarray, rather than (Bash:) the number of positional parameters, then the string'array[3]'.@gamma I just use them out of consistency and principle, so I don’t need to think in which case it is required or not.