Can't call API methods due to them being public
The following api methods do not work:

void API_SkinSign(BasePlayer player, Signage sign, string url, bool raw = false)
void API_SignText(BasePlayer player, Signage sign, string message, int fontsize = 30, string color = "FFFFFF", string bgcolor = "000000")

This is because they are defined as public inside the pugin while they need to be private. As can be seen here in the umod documentation. I am not aware of a git repository. If there is one I would be happy to submit a PR!
The plugin should change them to add [HookMethod] attributes if they are intended to be public for some reason. As an alternative, you could // Requires: SignArist and call the public methods.
Where does it say only private methods can be called? That's seems really unintuitive, so it should be documented explicitly.
5f1792699e67b.jpg WhiteThunder
Where does it say only private methods can be called? That's seems really unintuitive, so it should be documented explicitly.

That was how the original developer of Oxide designed it. Only private methods are callable via the Call/CallHook methods. Public methods can be called when using the Requires method.

It appears that the 2 API methods are public. In order for them to be called by other plugins, they must be private.