This is how dynamic variable scoping works in bash, and has always worked. The `localvar_unset' shopt is to provide alternate behavior for those applications that need it, but does not change the default behavior. The default remains as it always has for backwards compatibility.

Example (bash 4.4, debian buster):

function func2() {
    echo "func2 1: '${a-UNSET}'"
    unset a
    echo "func2 2: '${a-UNSET}'"
}

function func1() {
    local a
    a='local'
    echo "func1 1: '${a-UNSET}'"
    func2
    echo "func1 2: '${a-UNSET}'"
    a='trash'
}

a='global'
echo "global 1: '${a-UNSET}'"
func2
echo "global 2: '${a-UNSET}'"

a='global'
echo "global 3: '${a-UNSET}'"
func1
echo "global 4: '${a-UNSET}'"

What I expect:

global 1: 'global'
func2 1: 'global'
func2 2: 'UNSET'
global 2: 'UNSET'
global 3: 'global'
func1 1: 'local'
func2 1: 'local'
func2 2: 'UNSET'
func1 2: 'UNSET'
global 4: 'global'

What I see:

global 1: 'global'
func2 1: 'global'
func2 2: 'UNSET'
global 2: 'UNSET'
global 3: 'global'
func1 1: 'local'
func2 1: 'local'
func2 2: 'global' (!)
func1 2: 'global' (!!)
global 4: 'trash' (!!!)

So in some cases:
1. 'unset' do not do unset (!)
2. 'unset' breaks local variables of top functions (!!)
3. And then top function breaks global scope (!!!)

All problems are critical, because if 'unset' was success - variable MUST always be unsetted on the one hand. On the other - if you declare variable as local in one function (see func1), you  expect that you can't break global variable with the same name. In all cases - 'unset' MUST always unset just nearest local variable and never change it's scope. Because otherwise behavior of 'unset' is undefined (it shouldn't matter if you call func2 from global scope or from other function - the result must be the same)