A bit of preface. The games I run, the games I play in and the groups I enjoy all are very open with discussing meta. From story beats to encounter critiques and where we the players want to see the game go.

what do you do when the players go off-script?

We are honest and appreciate the time the GMs put into running the games. Several times either I or another GM have stated "that direction is not prepared" and the group have a chat from there. Perhaps calling it early or we zoom in on character daily life (or their projects). The amount of times a "forgotten" villain have reappeared for revenge in these situation is kinda high.

on railroading

Why play a game about characters, their decisions and their reaction to adversary when their decisions won't matter? When the roads they travel all lead to Rome? This is very much also something that is part of the game's setup. How directed the game will be. A wide open sandbox will strain much more against being directed than a more tightly focused narrative. Heck, I don't actually mind being directed in a game with a focused narrative or having the GM drawing the game back to it's story.

It's a complex topic where advice will differ depending on the specifics of each game.