There is an "enhancement request" in the system - SPR PHAN8R2GRE - for a spell checker in XPINC.
It looks as though this will be 854 at the earliest.
Not great for people that have "upgraded to XPages" Apps from Classic Notes Apps.
If you are in a position to raise a ticket with IBM please do so, the more weight the better.
Please also specify if you need spell checking while disconnected on a local replica.