For programming language/interpreter that's not true.
Lately I've been doing a lot of "self-hacking" on iOS and I'm quite satisfied with it. There are Python environments, some JS ones. Of course, they're not 100% desktop-like, but limitations aren't that different from various embedded systems. Those can be integrated into widgets/watch faces, on-demand and remote triggered scripts. There's also possibility to write webapps that run in full-screen form (good for calculators, small snippets, there's some iOS API to be used but I didn't bother that deeply to check). Some stuff can be ran completely, there's rich automation library and plenty more.
Could you give some examples? There is nothing for Java or Haskell, other than perhaps the almost killed iSH, but that is also a giant hack trying to balance on a thin wire. Apple didn’t take it down only because the huge backlash, which is not the case for smaller apps, unfortunately.
Most likely for Java and Haskell you won't be able to find anything. I personally used Pyto and Pythonista 3 for Python (there's also Juno for local and remote development if you're into Jupyter notebooks). I also use Drafts, which is editor, but uses JS for scripting and I and few my peers use it for automation and whatnot. I also seen Scriptable in use but I haven't used it myself.
As for other things - I'm Shortcut user and there are few extension to it like Pushcut or Toolbox Pro which extend it. I have shortcuts that manage my devices at home, when I'm there or when I leave, plus stuff I automated some with NFC stickers & Watch. Usually I start with it, when I want to get something automated. Shortcuts + servers provides a lot of automation (especially since I can get the data for widgets or make an action through it).
As for the other (especially compiled) languages I'm not sure if it's possible even possible without more advanced tools (and as far as I remember jailbreak voids warranty, but is confirmed to be legal). One can, however, create iOS applications without being part of developer program (this requires re-signing every week or two, which is inconvenience, but still something that can be automated) and for those 99$/year you don't have to re-sign every period. AppStore presence is not obligatory so one can make use of all sensors and data iOS applications have and even more (as AppStore rules don't oblige to your own personal apps) - and even share it to a small group of friends assuming they trust you, as they'd have to manually accept your dev certificate - something you can quite easily do with family or friends. It works for me but YMMV.
In general I'd say that for power user Apple's ecosystem isn't that restricted as people make it. On MacOS you get widely supported AppleScript that I personally despise due to syntax (and it seems like Apple dropped the idea of supporting JXA lately), on iOS there's reasonable x-callback/Shortcuts automation framework and quite a lot of commercial tools.
Afaik, JIT compilation is disallowed other than for Safari’s javascript engine. This is why x86 interprets instructions for example, which is more of a gray category.
Lately I've been doing a lot of "self-hacking" on iOS and I'm quite satisfied with it. There are Python environments, some JS ones. Of course, they're not 100% desktop-like, but limitations aren't that different from various embedded systems. Those can be integrated into widgets/watch faces, on-demand and remote triggered scripts. There's also possibility to write webapps that run in full-screen form (good for calculators, small snippets, there's some iOS API to be used but I didn't bother that deeply to check). Some stuff can be ran completely, there's rich automation library and plenty more.