Thanks for the considered response.
I would guess that if I knew what I was doing it would not be a big deal to add pspell to the Bitnami image of Simple Machines Forum.
However, that is what is great about the Bitnami approach. I can deploy SMF as a GCP instance without knowing anything about the operating system, the database, the patch levels, ... I can just focus on configuring and using SMF.
SMF is a forum where users post messages. Historically a server side spell checker was a key component, users could spell check their posts before they posted. With the advent of spell checkers within the browser and smart phone spell checking this requirement has become less important and some users of SMF do not use the functionality.
In my opinion the are still situations where server side spell checking is valuable. Care has to be taken to balance cost against functionality but for me there are situations where I just want to turn on the server side spell checker.
SMF has a simple tick box to turn on spell checking but, as I have found out, this does not activate software within SMF but instead turns on a link to pspell, but that must be already be installed on the server.
So I think it is fair to say that a minority of your SMF users would find it useful but it is not a high priority requirement.