Yeah, this forum isn't really designed for a nice filter. Probably not going to be implemented in the current forum software, I suspect it would break too many things, and be too time consuming to implement. The best your going to get is filtering it yourself via the search function which can be rather dire at times or using a search engine to do the filtering.
Simple Machines Forum while how good it used to be, really starts to show its age when it comes to implementing new features, due to the fact that it doesn't seem to scale well, it wasn't built with modern coding practices, and even the smallest change can break things. At least, that's been my limited experience with it.
Being able to filter posts by "keyword" efficiently would require a full text search index be added to the forum database.
My guess is that theymos has concluded that adding the above index is too expensive for the benefit it would provide.
Anytime you add an index to a database, it will become more expensive to add an additional row (such as a post) that is affected by the index. There are already a lot of indexs on the forum database that affects posts, such as the topic ID, the board ID, if a post is an OP, and the UID of the person making the post.
If there was a sufficient benefit, I don't think it would be especially difficult to implement. I am not particularly familiar with SMF, but I don't think it would be terribly more complex than automatically running a SQL query based on the text being searched, and creating an HTML template to display the results. The search text would need to be escaped, but that is true for all searches.
The above would at least be the solution if the forum was deployed via a flask app. It was
suggested that the new forum software be build on Django (which is very similar to flask). I am not really sure why this option was not used. A flask app that can host a basic forum could probably be built in a week or two. Some of the
additional required features might take a little bit longer, but only because they are so numerous. Each of the requirements would essentially be building SQL queries that do each of the required features, and possibly expanding/adjusting the database schema. Using flask or Django would not even require the forum using a server (except for its database).
