I think filtering would be rather complex since it would have to be implemented in dozens of parts of the code. Thankfully, Gramp has a feature that, in my opinion, is better suited as a “universal” filter, which would affect people, families and events: “Proxy Databases.” Fortunately, FTV already supports these with its “Presentation Mode.”
However, to achieve an “as-of date” visualization, a dedicated proxy would need to be implemented to check the dates of events of people and families, as well as handle edge cases, such as range dates and missing dates. The advantage of a dedicated “as-of date” proxy would also allow other views, reports etc. to easily support it to create outputs based on the specified date. Wherever there is proxy support (often labeled as a “living people” option), adding support for the new proxy should be easy.
P.S. According to my TODO list, you suggested this feature here back in December of last year. Now that filter support and the Presentation Mode have been implemented, it’s worth revisiting these types of feature requests. So, it’s appreciated when old ideas are brought back for discussion!