Cashtab was originally a create-react-app project. It has since been "ejected" -- meaning all of the hidden logic built into create-react-app has been converted into specific config files, such as webpack config. These files have been maintained and updated over time.
So, Cashtab is not really a create-react-app project anymore. However, since create-react-app uses ppular react and webpack dependencies, many of these persist.
Recently this warning appeared when running npm run build
Creating an optimized production build... One of your dependencies, babel-preset-react-app, is importing the "@babel/plugin-proposal-private-property-in-object" package without declaring it in its dependencies. This is currently working because "@babel/plugin-proposal-private-property-in-object" is already in your node_modules folder for unrelated reasons, but it may break at any time. babel-preset-react-app is part of the create-react-app project, which is not maintianed anymore. It is thus unlikely that this bug will ever be fixed. Add "@babel/plugin-proposal-private-property-in-object" to your devDependencies to work around this error. This will make this message go away.
Add @babel/plugin-proposal-private-property-in-object to dev dependencies so that it is not lost in the event of changes to the other dependency coincidentally requiring it (and thus allowing the builds to pass).