It is a bug in st and xterm. tmux and screen handle it by reflowing lines, wrapping them if necessary.
... And this tmux wrapping is thoroughly broken. E.g. https://github.com/tmux/tmux/issues/516
dvtm makes end of lines invisible [1] but keeps them in memory.
But this idea seems reasonable, as standalone from scrollback. No cropping ever. Ah, how it sounds... And what $COLUMNS will you report to term apps? Some of them (like ps) crop their output themselves. Some (like cat /proc/self/maps) align data columns on term width. So, desired effect still won't be achieved. But if you change $COLUMNS it will break much more of other apps. So, no silver bullet.
