Hi! Considering the example given in https://unix.stackexchange.com/a/648243/320598 I think it may be a bug, or an inappropriately documented feature.
Basically:
% x=(1 2 3)
% echo "${#x[@]}"; echo "${x[@]}"
3
1 2 3
% unset x[0]
% echo "${#x[@]}"; echo "${x[@]}"
2
2 3
% unset x[0]
% echo "${#x[@]}"; echo "${x[@]}"
2
2 3
% unset x[0]
% echo "${#x[@]}"; echo "${x[@]}"
2
2 3
