Tetra Lockscreen, one of many apps in the Microsoft Garage project, has evidently been removed from the Windows Phone Store. The app infamously did not work on the Lumia 1520, due to an incompatibility bug that was being worked on, but for other phones like the Lumia 830 it worked just fine.
Reports started coming in early February that the app had gone missing, but some of it appeared to be regional or specific phone related. However, now it seems that for all Windows Phones, the app is indeed gone.
No word from Microsoft on the reason Tetra Locksreen was pulled, or whether it will come back at a later date. Complaints about the app varied with some users experiencing severe lag when turning on and unlocking their devices (Tetra seemed to run better on devices with modern quad-core processors).
It was only at the end of January that an update for the app was released. That update brought Weather Beta, which was a useful interactive plugin for the lockscreen.
Hopefully, this is just a temporary pull until any issues can be ironed out as the app had a lot of promise.
Thanks, Mark R., Kris P., and Mike P., for the tips!