Neither of the "solutions" posted here actually turn it off ("Show inline predictive text" or "Show Siri suggestions in application"). It does not appear there's any way to disable it without turning off Apple Intelligence altogether (and I'm not even sure _that_ works).
These "suggestions" are awful. They're bland, are rarely the "answer" I'd give and _never_ the way I'd give it-- and have that irritating "shimmer effect" just to emphasize their awfulness.
Honestly, none of this AI stuff (Apple or otherwise) seems to work anywhere near as well as it's proponents claim, and I'm not seeing much downside for just turning it all off with the master switch at this point.