Referring to this question about "txt speak", is this on topic on Moderators?
If not, is there a more appropriate Stack Exchange site that we could direct this type of question toward?
I say yes*. It is related to moderation and is something that moderators have to deal with often.
However, I do not think we should allow these questions:
For that particular question, it seems slightly opinion based, but I think it could be salvaged with an edit (or two). The real question is Should I require users to use correct grammar?
The idea behind this appears to be how to get clear communications from users. To that end, it seems to be related to content curation and user behavior. Both of which are in scope.
As originally asked though, it does seem a little broad because "txt speak" is commonly used around the internet (Twitter, anyone?). It is accepted (and often required) there. Certain demographics are more prone to using this type of communication as well. Without information about what type of site it is or information about the users, I feel it's a little broad.
I do think communication standards question are on topic, but I don't believe this question is a communications standards question. It instead is asking "what are the reasons to use proper grammar?" This is distinctly different from talking about communications standards. As Annonomus Penguin pointed out, altering it to ask "Should I require users to use correct grammar?" is certainly on topic, but I'm not sure that the reasons to give as support for using correct grammar are on topic.
Put another way, a question about the reasons to have a policy that a community should speak clearly is related to communities, but this question appears to be asking for personal reasons to use proper English, which is not really related.