I'm very very VERY disappointed to discover this. I paid for the Haiku app and then again for the huge disappointment the Space app was. Now I can't even continue to use Haiku on the new phone... I'ts a pity since the horrible Space is still available, so the developer is still active on the iTunes store. I think everyone will be VERY happy if this thing can be fixed.
Also, since iTunes does not allow to install IPAs form the library on the connected devices I found another workaround using a specific versione of iTunes that still allows that and does not prompt to upgrade, developed by Apple itself: https://support.apple.com/fr-fr/HT208079
NOTE: you have to uninstall the newest iTunes version, and the old version may complain that the library is no longer compatible. Using shift while clicking on the app will prompt you to load a library or to create a new one. The second option worked for me but I don't keep music or anything on the phone.
We restored, via iTunes, another iPad from a backup that had Haiku and ran into the same issue. Would not install. Thank goodness we have switched to Myro:Home which frankly is superior (and fully supported) IMHO. The Apple watch app integration is also really slick. Certainly one option for folks, but understand their is an ongoing cost. I’ve been burnt several times by one off apps that don’t have a subscription and eventually die because the business model to support these unique apps doesn’t work. SJ