We are aiming to provide network security (e.g., pishing, malicious sites). While this can be done on Safari via Safari extension, it requires managed app solutions for third party browsers. Supervised device restriction is too much for most users, but many of our customers have MDM so managed apps are ok. Unfortunately, when testing it on managed apps (chrome, firefox) we don't get any NEFilterBrowserFlows, so we can't show a blocking page. Is someone checked this? seems weird that content filter can't show a blocking page only drop a connection.

True, but we aim for iOS 16 and above. Checking on both iOS 16 and iOa 17 beta with per app content filter we get no browser flow at all, which prevents us from using remediation to show a blocking screen and thus are kinda useless.