Full immersive still isn't possible, the status bar ain't going nowhere with their customisations.That's pretty-much what this module does, immersive mode on Android 11. I do use Nova with a hidden status bar so I don't know what this mod does with other launchers but it gives me full immersive mode in landscape for games, YouTube, Netflix etc, etc.
Edit: I re-read your post and you're asking specifically about portrait... do you want that for all apps or just the launcher? Based on previous discussions I recall most people want landscape only immersive mode.
I tried many days to find where they force the status bar to always reserve space in portrait but had no luck. Closest I got was to disable all the oneplus overlays but then the system started behaving strangely