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 log.
But the option is missing completely from the sequencer or any other place.
I think it is bugged or something.
There was not much difference with it enabled or disabled, there was frames skips with scenes having very low fps (1-2fps).
What fixed the issue for me was rendering it at 240 fps.
