As did I, maybe a PR to remove it from Geany? [ducks] 😁
Geany is a lightweight IDE with a feature for auto-closing built in. I don't think a thing like automatic inserting closing characters should be abandoned as it's not an text-editor, it's an IDE.
The argument that any specific functionality should be built-in, not in plugins does not make sense. All current IDEs/editors use plugins to add/extend features, it allows different use-cases to be addressed without arguments about what should be built in, the user just loads the plugin/extension that suits them.
As far I'am concerned I don't know an easy way inside Geany like an plugin-store to install plugins/extensions as in e.g. Chrome, Firefox, VS-Code or Eclipse. Even more inside Geany there is no hint, where plugins can be got from. Furthermore it may not be possible due to organisation/company restrictions to install anything else than Geany without plugins. So leaving "core"-functionality in an broken state with the reasoning it can be fixed by a plugin is short-sighted.