I am currently looking at using Microsoft Web Chat to interact with my existing Node Js application built on MS Bot Framework Core but uses DialogFlow as NLU.
Current implementation: Web Page -> Custom UI Widget -> Connects to Node.js app built on MS Botframework Core -> Queries DialogFlow NLU to identify Intent -> Node.js app constructs the Dialog -> Gives response to user.
Looking at the Web Chat component, I cannot find references using any other NLU except Azure Bot Service. Is it mandatory to use Azure Bot Service/LUIS NLU to integrate with Web Chat?
I have also looked at offline-directline npm module but the last dev on the module is more than 3 years ago.
Firstly, responding with a quick message about using offline-directline, you should feel reasonably comfortable with it. It is a few years old, but seems to stand the test of time with continued use and little in the way of issues that I have heard.
Another option would be to use a 'browser bot'. In this scenario, the bot is contained within the hosting page's html/scripts. So, there is no reliance on using Direct Line. I don't know your whole setup, so this may or may not align with your overall needs and architecture.
As for using Web Chat with something other than the Azure Bot Service, this would be an uphill battle. In theory, this is probably achievable to some degree. The issue is that Web Chat is heavily integrated with the BotFramework-DirectLineJS library. You might be able to cherry pick specific components from Web Chat, replace the Direct Line library with a make of your own, and modify the remaining code to work with your 'service'. But, I'm not certain the effort is worth it. That is something you would have to decide for yourself.
If you do go this direction, you will likely need to configure your 'service' to send messages that conform to the BotFramework schemas when communicating with Web Chat. Even without the dependency on Direct Line for connecting to the service, much of Web Chat is still oriented towards what an incoming message looks like (i.e. a BotFramework Activity) and handling it according to the properties it contains.
To start, here are a few areas you should review in order to correctly configure you service to handle inbound and outbound messages going to and coming from Web Chat:
This is not exhaustive and would require greater research as there are likely other considerations than the few I've listed above. But, again, this may not be necessary if you decide to utilize offline-directline.