Is this still how scripts are written? Feels like not being able to figure out an ending is something that was pretty common up until the 1970s, usually with the script of an otherwise great film just getting weird in the last 15 minutes as a result. I figured this was mostly a typewriter limitation, where editing was a lot more expensive.

For example, 2001's and its star child weirdness, The IPCRESS file, and many others.

Seems more often scripts are written with an ending in mind nowadays, with the weird bandaids ending up in the middle instead.

Maybe a bit OT in an article that's trying to be about AI but...

Yes, modern screenwriting classes hammer home some variation of the five-act structure and, the particular beats to hit at each point. It's rare for any narrative film, even indies, to deviate from it much, and you are absolutely told to map out your whole narrative and know where it's going before you begin.

I'm sure there are some screenwriters who ignore all that and just start writing. Particularly if they're experienced enough to have an intuitive grasp of structure. But if you're a first time writer and reach the night before a submission deadline and you haven't even finished the first draft, then you've got serious problems. Leaving aside the ending, any script needs multiple revisions with time in between so that you come back it with clear sight.