It is not like you paid for it. I don't think that it was successful. The only revenue would have been people buying planes. I have it on my iPad. I have only used it a couple of times. If the software is not successful, it gets removed. That is the way of capitalism. Personally, I think that RF-X will resurface again. My OPINION ONLY.
I kept the latest .apk file for the Android version. Maybe there is such a thing for Apple products. You could then find a place to download it and manually install it. I think it went the way of RF-X.
I looked and it's still available for Android in the Google Play Store.
This is the old link to the app on the Apple store. I don't use iTunes so no idea if it would still show up. But nothing to lose trying:
https://apps.apple.com/au/app/realflight-mobile/id872641883