How do I restore a deleted app that is not longer available on AppStore?

If the developer has removed it from the store (or Apple have removed it if it's no longer supported or violated the store's terms) you won't be able to redownload it, you would only be able to get it back on the phone if you have a copy of it somewhere e.g. on your computer or backup of your downloads/computer.

Side loading and other unsupported means of installing apps is not really an appropriate response to a user though.

Unless someone is really savvy about that, and knows well where they are getting installers, they could cause themselves a lot of problems. Not to mention using software that may be distributed illegally.

Yes, I am sure when they were available in the Apple App Store they were indeed legally bought. That does not give you any legal right to download them from some pirate software site and side load them onto your device. If they are no longer in the Apple App Store, then any web site offering installers for download is almost certainly not legally distributing those software. Nor can you trust that whatever you download and side load from some random web site is in fact free of malware, spyware or other malicious code since side loading by definition bypasses all of the normal iOS and App Store install checks and security.

That is what Asher Menezes is talking about. It is neither recommended nor supported by Apple.

Do you have a copy of it on your computer so that you sync/copy it back ?

Go to

App Store -> Updates -> Purchased -> All / Not on this iPhone

Navigate in either of the columns and you should find your App !

For your information I have managed to download Apps I couldn't find on the App Store ! and still use them with no issues!

The answer is simple. If it’s been pulled from the App Store, you cannot get it. You may not be happy with that answer but it’s an accurate one.