UPDATE: I should have added that this change does not impact Windows 10 Mobile: as with Windows Phone, you can still install apps to an SD card with Windows 10 Mobile. When I refer to Windows 10 here, I mean "Windows 10 for PCs and big tablets," or what I sometimes call "big" Windows. –Paul

While testing small tablets with Windows 10, I noticed that it wasn't possible to install apps to an SD card rather than internal storage. Well, it turns out this is by design: Microsoft has postponed this functionality "to a later release."

This could be a huge problem for users with small tablets and other PCs that have minimal amounts—16 GB to 32 GB—of internal storage. The ability to push as much content as possible off to an SD (or microSD) card is what made many of these devices feasible in the first place.

Fortunately, you can still configure Windows 10 to use an SD card for documents, music, pictures and videos. You can find this interface—and see that "New apps will be saved to" is grayed-out—in Settings, System, Storage.

Microsoft made this revelation in a support forum.

"We have actually made the decision to postpone the apps on SD functionality to a later release. While I know this feature excited many of us, we didn't feel like it was ready for prime time. Our goal is to delight users with everything that we build, and we truly feel that that is what will happen once we have fully built our vision for this feature."

Thanks to @sourcegrinder for pointing this out to me on Twitter.