As the sibling comment by eternityforest says,
OPFS does not create real files on your file system. It is backed by sqlite or some other embedded key value store.
So, if you want to backup your webapp data, you cannot use any filesystem backup tool.
And since OPFS cannot access your local files you cannot use it for webapps that largely use local file system data.
Something like scoped storage would have been great. Each webapp gets a separate directory where they can store your data. Easy to backup and you can also drop any local files or directory in there with existing OS tools.
So, if you want to backup your webapp data, you cannot use any filesystem backup tool.
And since OPFS cannot access your local files you cannot use it for webapps that largely use local file system data.
Something like scoped storage would have been great. Each webapp gets a separate directory where they can store your data. Easy to backup and you can also drop any local files or directory in there with existing OS tools.