What possible data can I lose if I backup iPhone to iTunes and then restore?
What is not included in the iTunes backup itself:


Media (synced with iTunes but not in the backup). Videos, music, podcasts, etc.
Books (can be synced with iCloud or exported with third-party iTunes replacer if stored locally). Includes both ePub and PDF stored in iBooks app.
Apps that are no longer on the AppStore and not accessible via Purchased > Not on this phone (as you've already mentioned). But app
data is still saved in the backup and can be accessed with
third-party apps.
App data of some apps. App developers decide what to include in the backup, so be careful with messengers like Viber or Signal (they
can have a separate backup way, check google), or with important
docs. Media players usually don't include tracks/videos as well.
Some data that is synced with iCloud. For example, if you have iCloud Photo Library with "Optimize storage" enabled, iTunes backup
will include only thumbnails of photos if full-size version is stored
on the cloud.
Emails.
App cache (most of the "Other" data).


Should be all if not mistaking.