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]’.