I have an internal search in my web app for text with filtering by some type of order and/or by multiple topics (like tags). If the tags are not present in the system there is no sense in filtering by tags, so I remove the filter.
How can I manage the appearing/disappearing filter? If I make this filter appearing / disappearing based on the existence of tags, I find some inconsistency for the user.
It sounds like this is not "rolling out of a feature" in the normal sense, where you might want to direct users' attention to something new they can do. Instead you are talking about something users have already been exposed to being invisible in some cases, and this is a different problem.
Luckily, its an easier problem, as there is no need on your part to advertise the change and have users visit some new part of the interface. Instead, all you need to do is explain the change (otherwise users will have to figure out the change on their own, and may conclude that it just stopped working properly).
I would suggest that, for instances when the tags are not present, you have an explicit message about no tag filtering being available in as close to the same position as the tag filtering UI element would be if tag filtering were available.
Some further, clarifying questions:
Generally, yes - you are correct, you want to avoid having UI appearing and disappearing without proper reason and context given. And that's the key here: context and reason. Sometimes it does make sense to hide functionality if it is no longer applicable to the user's new context. If that happens though, be sure to visually guide the user into the UI's new state. Give reasons to the user so they understand why things have changed. Basically, don't break the mental model that you've worked to establish with the user.
