Hello,
After the last update, the skin “confluence” no longer appears.
Need your help if you want.
PS: Sorry for my bad english 
Install it from the Kodi repository.
I do not succeed, I am completely lost, why have I removed confluence?
As said, go to the skin options, choose “More” and install confluence from the list. Then activate it as the default skin.
As to why confluence is not preinstalled anymore:
Kodi now has a new default skin called estruary.
The osmc skin is choosen as the default one to prevent any incompatibilities with the old skins on the new version of Kodi.