Is it possible to hide this button when you can’t actually use it? Thought that the standard installation would include this, however, it does not. Seems like a no brainer. Doesn’t seem like that’s an option, so this is a feature suggestion.
Sometimes being able to see what they can’t do can help advertise the feature to them. For example, if they would be able to do it if they joined a group or paid a subscription or used the platform more so they had a higher trust level, then showing a disabled button (or link or whatnot) can help them become aware the feature exists.
But I agree that it’s not always helpful to show such things.
One thing to consider here is user experience. If they see the button disabled, it’s pretty clear that for some reason they can’t create a topic. However, if it’s hidden, they might spend a long time looking for it, thinking it must be somewhere else or that there’s some trick to it!