There are many alternatives to Allo for iPhone if you are looking for a replacement. The best iPhone alternative is Joplin, which is both free and Open Source. If that doesn't suit you, our users have ranked more than 100 alternatives to Allo and loads of them are available for iPhone so hopefully you can find a suitable replacement. Other interesting iPhone alternatives to Allo are Standard Notes (Freemium, Open Source), Google Keep (Free), Microsoft OneNote (Freemium) and Evernote (Freemium).
Joplin is a free, open source note taking and to-do application, which can handle a large number of notes organised into notebooks. The notes are searchable, can be copied, tagged and modified either from the applications directly or from your own text editor.