Good to know about security guards not making the cut before people got hopes of them being there.
If i have to be frank, security guards have ALWAYS been the less interesting staff from a gameplay and visual standpoint in not only RCT 1 to 3, but also games like theme park series.
Vandals never really existed in these games, it was just something behind the scenes, you get too many people in the park and things start to be broken, there were not individual guests that broke those things, only an illusion of them being there. When you see this brake things, you just plop a security guard and they went they merry way going through the zone you indicated and think never broke where they walk.
You would never see a scurity guard throwing a guest out of the park or anything similar. They just walked. And I can tell you, security parks is not something you actually see in daylight at real theme parks. They mostly use cameras and people under cover (and real security staff was inside of a building monitoring this cameras), not actual security guards dressed walking through a park.
I understand that when Frontier came to the game design of them (and it was a sure thing at first glance becuase, of course they were in the older games), lots of questions starting popping up from a gameplay standpoint that couldn't be resolved as easy as it seems (creating games is resolving problems of things you start to add, "does it work on every way possible and if working is it fun?" is the mantra of game designers). Do they work like in old RCT games? In the much more creative game that is Planet Coaster, invisible vandals breaking lights and benches is not fun, is not as easy to change this super decorative things now as it was in RCT once they were broken. Then how can we make them more interesting and fun to play?
They probably tried a lot of new implementatios and nothing seem to work well for them, so the best thing was axed them for the time being, who knows, maybe they find something that works much better in the future (this happens a lot during development and a good time sleep).
Maybe they already tried this, but here is an idea for the future.
Instead of vandals breaking things or them being "invisible" they are actual people that start entering your park after you have a cosiderable amount of them.
This "vandals" wil distract entertainers and other guests, and entertainers and guests will start asking for security.
Security staff then comes handy to protect this conflict zones, if security catches this vandal distracting people, it automatically boots him out of the park and guests and entertainers are happier. Vandals will reapear in your park if you have a big amount of people coming in again and displease in queues, not founding food stalls or restrooms, or too little rides for the amount of people starts becoming rampant.
Or something like that. Of course, Im sure they already thought of something similar but you never know.
Would love for them to return in the future better implemented, of course, but its not something Im going to cry about.
EDIT: I actually think they were vandals hitting mascots in the original Theme Park.