Nice guide! I wish there
was something like this for apps that have been completely erased from AppStore (not even available under the purchased history). Afaik apart from jailbreaking there is no way to install those.
https://rderik.com/blog/resigning-ios-apps-from-an-ipa-for-m...
The downsides are that you can only have 3 sideloaded apps at a time, and you have to reinstall them every 7 days.
https://sideloadly.io/ automates the process and can automatically reinstall them for you - I use it to keep the Pebble app installed and my watch synced.
An app that I use daily got aquihired and was removed from the store. The company who made the app warned us in advance they were shutting down, so many of us were able to make a backup.
Or check trusted sites like apkmirror to see if they just have one.