Is a Nintendo 64 controller required to perform the Backwards Long Jump on the Nintendo-Switch-Online version of Mario 64? I’ve been trying with the Joy-cons, but can’t seem to get it. (That being said I’ve never done it before either). (Specifically I’ve been trying to do it on that small stair way to get into the second bowser fight)

I'm pretty sure that the SM64 version on switch the BLJ it's patched.
Also this trick u r trying to perform it's very very hard, you should try it on the infinite stairs to see if u can perform BLJs on this version.

NSO version is not patched (unlike the 3D All-Stars version). BLJs are possible with any controller, although I imagine it's harder on joy cons. BLJs are frustrating but assuming you have watched a guide you'll get it soon.

You'll also get better answers/help in the help channels of the sm64 discord server. It's better than asking questions here on the forums: https://discord.gg/0Si1OtnB7Ylmwkwr

I performed BLJs in handheld mode as well as using a pro controller. As long as you are in the NSO version its possible with any controller.

Yes all bljs are possible using any controller, for a couple months I did 1 star runs using joy cons until I got an n64, and I can tell you lblj and sblj are 100x easier using an n64 controller or really any controller with notches.