This was their response to me as of a week ago.
Thanks for getting in touch. At the moment visiblity conditions for slides do not work for the slider widget as that widget is not fully cms enabled yet. This means, it is not possible to suppress a slide, that you want to have not visible if there is not dynamic content due to a filter, condition, or non-existing content
There may be some workarounds until we add support for the cms and the slider, to be able to use dynamic features, but I do not have a timeline for this yet.
Here is a post that may help: Creating a Dynamic Slider in Webflow
Here is the dynamic slider item on the Wishlist: CMS Slider | Webflow Wishlist
Help for custom workarounds is provided on the forum, so if you can create a post about this and send me the link, I am happy to take a look.