This isn't really a "state of the web these days" complaint, it's what happens any time you ship cross-platform code. Look at any reasonably mature C/C++ codebase and you'll see plenty of '#ifdef _linux...#ifdef _OpenBSD...'.

Having complete control over the environment your code runs in is the exception, not the rule, though the modern trend for SaaS might make you think that backend code looks cleaner than the frontend.