Looking at it again, to me this sentence is nonsense (i.e. I can't make sense of it!)
[Edit, since indentation didn't work as I had expected: I mean the sentence beginning "Since Version 1.36, MediaWiki only commits to supporting upgrades from two LTS releases ago..."]
Looking at task T259771, it says: "Also, everything up to 1.35 can still be done in one step. So someone who, in four years, wants to upgrade from 1.29 to 1.43, will have to do two steps: one to 1.35, and one to 1.43. To upgrade to 1.50 in seven years, you'd go from 1.29 to 1.35 (2020), then to 1.43 (2024), and finally to 1.50 (2027). Not too terrible, I think."
So it looks like you could go straight from 1.15 to 1.35. You couldn't go from 1.15 to 1.36 though. You might as well go straight to 1.35, then 1.36, as then you don't have to mess around with different PHP versions etc.