I have seen this done for a few web apps I use. The ones that come to mind are Basecamp and Grammarly. Their landing pages basecamp.com and grammarly.com. Apps are redirected to 3.basecamp.com and app.grammarly.com.
To me, this felt just like a cool thing to do. However, once I was deep into developing Kushim it became frustrating to keep everything under the same codebase. The logic (HTML) and presentation (CSS) were too complex.
Separating the two seemed scary at first, but ones I have attempted to do that it was actually simple. It only required a couple of changes on the Nginx server. I will not go into detail here but will write a post on my website.
I am not sure if this was the right step to do. What I can say though is that I am now much more relaxed and feel that my ideas are more doable. Personally, I find it weird that these changes make me feel calm.
The fact that this was done out of necessity and provides the quickest result, feels very Indiehackerish. That is great.
I am now considering rewriting apps design from scratch and am leaning towards TailwindCSS for that. I am also going to use a service to help me build a landing page. I am considering Carrd right now.
I am sure I will have more things to share with the community in the nearest future. This change helped me to be positive about the future of my app.