Recently a member of the community made a video of one of my games, an extremely hard one level platformer. Throughout the vide they only made it about 1/2 way through the level, just passed where I would put a checkpoint if I was to add one. I was thinking with that checkpoint they might have gotten much further and maybe all the way to the end. It is possible to get to the end all in one shot, I almost did it myself, I made it all the way and then was killed by the very last hazard.. I do like the retro no check point style and also recognize that is more rage inducing, however I think due to the length and difficulty of level and the expectations on games, maybe I should add one. What do you think? Checkpoint or no checkpoint?

There's a difference between 'difficult' and 'defying player expectations.' This appears to be what's often considered a 'rage game,' and would probably attract a specific type of audience from most platformers, on its own. This makes it hard to speak for, on the matter of checkpoints - I've seen well-known titles in this niche, such as 'I Wanna Be The Guy,' which have save point. Nevertheless, how they're dispersed through the game presumably is based on differing design processes from standard platformers, so I'd recommend taking from similar titles, if you can't decide yourself. My opinion is that rage games do have to maintain some form of balance in this regard, at least, to ensure players who are into them don't actually ragequit.

For this instance in particular, I can't tell if they're bad at platformers, or if the controls suck somehow. Assuming the difficulty is sustained equally throughout, I could probably complete it no-checkpoint without too much trouble, if I really wanted to.

I always find the best formula for rage games is the "troll" level-style game, where comedy is the central point. The game can have impossible-to-predict deaths (totally unavoidable the first time through), but after learning from mistakes you should have a clear way to progress, or even supply hints if too many deaths to the same troll occur. Checkpoints after every few minutes of trolls is also a good design, and sometimes even scaring the player with knocking them back to an earlier checkpoint is good for laughs (provided you don't actually send them back without a new path opening up) - in Mario Maker terms, this is the "CP1" gag.

I’d add one checkpoint around the halfway point. It keeps the challenge but makes the run feel less frustrating.