FeelsBadMan

I read that you unlock it if you beat it in under 3 hours and don't save anybody (Get the worst ending on any respective character.) I haven't confirmed for myself yet though!

I think the requirements are different from what people say.

Since this is based on the 90's PC port:

https://www.youtube.com/watch?v=3Cfnl6Bt8_o

at 31 minutes in he gives info on unlocking the rocket launcher I would be inclined to believe him since he dives pretty deep into all these games, whole video is probably worth a watch also for someone interested in all the ports.

Yeah I think this is the correct answer. I just did a playthrough as Jill after watching that video, did not save the progress, and got the bad ending just to be sure, and I ended up unlocking the infinite rocket launcher.

I believe you have to do a no save run and time doesn't matter.

Just check this topic that explains the exact requirements:
https://www.gog.com/forum/resident_evil_series/infinite_weaponsbonus_items_re1

For the special weapons, ingram and machinegun, you need to finish the game under 4h

For the infinite rocket launcher, you need to finish the game without saving (no time limit I think, can also be done using the special weapons from finishing the game under 4h)

For the special key, need to finish the game with the good ending

You gotta do a no save run and time doesn't really matter.