Is separating frontend and backend always necessary in modern apps?

In a lot of traditional architecture discussions, separating frontend and backend is treated as a default best practice.

But with modern tools like Next.js, server actions, BFF patterns, and full-stack frameworks, that separation doesn’t always feel as strict anymore.

In some cases, everything lives in a single codebase and even shares logic between client and server. In others, teams still prefer a clear separation for scalability, team structure, and maintainability.

Is separating frontend and backend still a necessary architectural choice for most modern applications, or has the line between them become more of an optional design decision depending on the project?