The component is defined in webpack/assets/javascripts/react_app/components/common/forms/CounterInput/CounterInput.js

It uses rc-input-number library as it's base. Even though the library seems to be still maintained, it would be better to get rid of one more dependency, especially given that PF5 already has similar capabilities: https://v5-archive.patternfly.org/components/number-input/html