Because we use backslashes and not normal slashes, this escapes the complex string templating and results in the wrong filter name:
$strings[ $key ] = apply_filters( "air_cookie\strings\{$key}", $string );
with key consent_modal_title results in a filter "air_cookie\strings\{consent_modal_title}" being applied.
Proper usage would be
$strings[ $key ] = apply_filters( "air_cookie\strings\\{$key}", $string );
As seen on PHP String docs "Complex (curly) syntax" in the last 5 lines of the first code example.
// Won't work, outputs: C:\folder\{fantastic}.txt
echo "C:\folder\{$great}.txt"
// Works, outputs: C:\folder\fantastic.txt
echo "C:\\folder\\{$great}.txt"