Same here, I was just getting some comments on my blog posts and couldn’t find a reply-button.
Can’t be, right? I am just not seeing it, right?
Asked Google, now I am here and kinda furious why this really a thing and after so many years that Shopify exists there still is no such (simple) feature.
It is really a big downer when you cannot reply to someone and the person does not even have the chance to be notified when I reply.
Posting a new comment on the front end is kinda hilarious in 2020 
Please add my voice to the “Reply-Function”-Counter, maybe it will help push this feature a little bit more to the top 