The option called force fixed frame rate interval used to render sequences that had very low fps smoothly at a give frame rate.
I have scenes where fps hits 2-3 fps but using the option in sequencer, it allowed me to render videos at my desired fps.
With 4.20 I saw this in the change long.
I was doing panorama recording that took 1-2 mins per panorama. And after I have tried tried rendering a video 5 frames long, the result was 2 panoramas each time instead of 5.
What did work for me was increasing the rendered frame rate to 240.
