Introduced in iOS 9, Universal Links allow iOS developers to claim ownership of domain names (including wildcards) that can be processed by that developer's iOS app. When an iOS user taps on a link to a URL of that domain name in any app, such as Safari or Mail, and the user has that 3rd party app installed, that 3rd party app is immediately launched to service the URL.
For web browsing apps on iOS that route traffic through VPNs or Tor, this feature can cause traffic to be sent outside of the VPN/Tor network without warning. For instance, if one has the eBay app installed and taps on this link from within Safari or any other web browsing app on iOS, the eBay app will be opened to load that auction page.
Shortly after the initial release, I received some great feedback from Chad Etzel, one of the creators of Notifo, the notification service that I used until it was shut down (which prompted me to create Pushover in the first place). Chad asked for Pushover to support sending messages with URLs that can open external apps, and Pushover soon gained supplementary URL support which required changes in the API and on both Android and iOS apps.
On March 7th, 2012, I announced the launch of Pushover, a simple mobile notification service with device clients available for Android and iOS. I kept some notes during the development process, which mostly occurred in the evenings and weekends around my other work.
I had been using Notifo for a year or so to receive push notifications on my phone from my custom network monitor, but last year the free service announced it was shutting down. When I switched back to my Android phone a few months ago, I was unable to download Notifo's Android app which never made it out of beta.