echo "Ninth parameter: $9"
echo "Tenth parameter: $10"

echo "Ninth parameter: $9"
echo "Tenth parameter: ${10}"

For legacy reasons, $10 is interpreted as the variable $1 followed by the literal string 0.

Curly braces are needed to tell the shell that both digits are part of the parameter expansion.

If you wanted the trailing digits to be literal, ${1}0 will make this clear to both humans and ShellCheck.

In dash, $10 is (wrongly) interpreted as ${10}, so some 'reversed' care should also be taken:

bash -c 'set a b c d e f g h i j; echo $10 ${1}0' # POSIX: a0 a0
dash -c 'set a b c d e f g h i j; echo $10 ${1}0' # WRONG: j a0

• BashFaq: How can I access positional parameters after $9?
• StackOverflow: How to handle more than 10 parameters in shell
• Autoconf Manual: Shell Substitutions - documents some non-POSIX older shells too