For the Wiki app, there are branches for v13, v14, but not v15 yet.
With Frappe app released version-16, is it still safe to use the development branch for the Wiki app (on a server running frappe version 15)?
Be careful using the develop branch on a production v15 server, as it might have changes for newer Frappe versions and could become incompatible with v15. If you must use it, test on a separate v15 bench first, including migrations and your essential Wiki features. Pin it to a tested commit instead of automatically pulling the latest changes. It’s also a good idea to check the Wiki repo’s recent commits or ask if a version-15 branch is planned.
“Be careful”?
This is my exact concern. Does Frappe have any plans to make a v15 branch for WiKi?
This issue was raised back in March. It seems Frappe is not interested in stability, or they feel there are no breaking changes… which I’ve already seen plenty.
The stable branch is version-3, which works with version 16 of Frappe. The main branch works with version 15. We will update the README soon.
Starting version 17, the branch names will be made identical to Framework/ERPNext, i.e. version-17.
As for Frappe Wiki v3 support for version 15, we will see if we can make it compatible next week 
Track issue here.
cc: @gajendra
Yeah, that’s reasonable - by “be careful” I mostly meant that without a v15 branch there’s no explicit compatibility guaranty for the develop branch.
I don’t know if Frappe has intentions to establish a v15 branch too. I’d be wary of using develop for a production v15 configuration since #585 has been open since March without a clear direction until someone from the Wiki team can validate it works.
Version 3 (version-3) branch now supports Version 15 of Frappe Framework!
Starting this release: Release v3.4.0 · frappe/wiki · GitHub
Credits to @gajendra