Prusa MK3S Y-skewed prints
Apparently, the possible cause mentioned by @R.. GitHub STOP HELPING ICE turned out to be true. The frame was noticeably skewed in the Y-axis, which is why the prints were leaning in that direction.
I replaced the Z-axis top parts with a 3D printed modified version of them (what came with the printer is orange, what I printed is yellow.) The modified version is slightly longer so as to move the steel rods (ones that hold the X-axis) slightly towards the front of the printer (in the opposite direction to the skew)
This turned out to work perfectly, the skew is still slightly present, but it has been substantially mitigated and I can fix it completely by adjusting the length of the printed parts.