Good game, Mr. Rum. Here is my personal feedback:
I really like the insane momentum you get when you grapple towards a point and let go early. I feel like this is a mechanic that should be further accentuated by the level design if you ever choose to update the game. It's really fun to gain speed that way. I really like this mechanic a lot.
However, I dislike how the player stops completely still once they arrive at the position a the grappling point. It feels quite clunky. I suggest that the game instead forcibly exits the player out from their grappling state and allows them to continue in the direction they were moving with all the momentum they've built.
The game is also insanely difficult.
The bit where you've got to grapple over the wall expects the player to be too precise and, with the aforementioned sticking to grapple points, it to me feels clunky.
With the pit area afterwards, with the vertical purple point then green grappling point, it felt unclear what the game wanted me to do.
I assumed the game wanted me to gain downward momentum by grappling to the purple point, then to use this momentum to swing across the spikes via the green point. I really liked this idea, but if this was your intention, I recommend replacing the spikes below with a pit - this would give the player more room for error. In addition to this suggestion, I had left you a little rant, but then I eventually realized that you have to cancel the second swing early. Took me way too long to figure that out. Maybe you could put an easier obstacle based around this idea earlier in the level to help the player figure that out easier? Spit balling here.
The following obstacle was also insanely painful. It's hard to tell exactly where the player is going to be sent when grappling, so it was hard to be so precise. I feel like you've got to give the player more room, because beating this feels like just luck compounded by muscle memory. I tried really hard to beat this, but eventually I just had to rage-quit.
Anyways, it probably sounds like I'm berating you, but I liked it enough to play it for around an hour or more. I enjoyed the ideas behind the obstacle designs. Ultra punishing difficulty might have been what you're going for. I just think it needs to feel more fair to the player. I've also never designed a platformer myself.
Anyways, Mr. Rum, I hope you do end up making more.