Do not exists a real need to include it with GOS at all.
I disagree, the following is a quote from the founder of GrapheneOS regarding the AOSP keyboard: (from the same github issue posted above)
Sticking with outdated code that's not being maintained is definitely not good for security, especially since it has a bunch of code unnecessarily written in C++ rather than Kotlin/Java.
And
Attack surface would be reduced by getting rid of C++ code and instead having almost entirely Kotlin/Java code that's memory and type safe.
So in my opinion, there is a need. I don't care honestly with what keyboard it would be replaced, but a replaced keyboard would mean it passed GOS audit, and an improvement securitywise.