Deprecated means it is fully functional but may be removed in future versions. Google of course prefers that their cloud is used so that they still control access to the backup data.
The only part that seem to have changed by the deprecation is that the help text of
adb backup has been removed from the adb binaries. So to show the help text with all options of adb backup you have to execute now:
adb shell bu help
adb backup has a problem with app that disallow backup via it's AndroidManifest.xml. The main problem you don't get any warnings or errors for such apps. hence it is totally unclear if a backup was successful by just looking at the messages printed on the console while creating the backup.
From an app perspective there is also the way to allow only partial backups. By a configuration file the app can specify certain paths or file types to be included or excluded from backup.
Therefore my recommendations is to backup only single apps without APK. Based on the size of the created backup archive you can see if the backup was potentially successful or not.
An alternative is to create a full backup (with or without APKs) and in the end convert the created backup archive using Android Backup Extractor to a tar archive and check that archive for the most relevant apps what files have been backuped.