but Geany uses them when they're in tagmanager format
I don't think tagmanager actually handles parameters as such, it handles the whole argument list as a single string IIRC so you can't access the parameter names.
The new C/C++ parser can parse locals and parameters IIUC, see #3032 and associated discussion. Geany (and ctags??) has no way of specifying scope for these because the scope is lexical not named like structure and class members. So they appear everywhere, not restricted to the appropriate function. So #3032 disables them by default until that problem is solved.