That button has 3 states built-in:
- Logged out: links to login
- Can start discussion: normal button
- Cannot start discussion: disabled button
It's usually expected that when the button is disabled, it will become enabled when you change tags or after you enable your account. It would be odd if the side navigation was jumping around when you change tags because a new button appeared.
If your forum has a situation where some users will never have access to the button, then using the extension API to hide the button completely could make sense. It might even be doable just with CSS, looking for the disabled state so it stays visible to allowed users.
Can I ask about your use case? Are you only allowing your members to reply to discussions but never create new ones?