That's not a problem in a well designed ASP.NET project. Just create a new web API project, move a controller into it and copy/paste the necessary boilerplate into Program.cs, set up config etc for it and configure cicd to deploy it separately, there you go. Less than a days work.

You can also publish libraries as nuget packages (privately if necessary) to share code across repos if you want the new app in it's own repo.

I've worked on projects with multiple frontends, multiple backbends, lots of separately deployed Azure functions etc, it's no problem at all to make significant structural changes as long as the code isn't a big ball of mud.

I always start with a monolith, we can easily make these changes when necessary. No point complicating things until you actually have a reason to.