The answer you have is (nearly) correct for non-sparse indexed arrays¹:
unset 'arr[${#arr[@]}-1]'
Bash 4.3 or higher added this new syntax to do the same:
unset arr[-1]
(Note the single quotes: they prevent pathname expansion).
Demo:
arr=( a b c )
echo ${#arr[@]}
3
for a in "${arr[@]}"; do echo "$a"; done
a b c
unset 'arr[${#arr[@]}-1]'
for a in "${arr[@]}"; do echo "$a"; done
a b
Punchline
echo ${#arr[@]}
2
(GNU bash, version 4.2.8(1)-release (x86_64-pc-linux-gnu))
¹ @Wil provided an excellent answer that works for all kinds of arrays