I've found in my area, event talkgroups are used for the same purposes year after year. Say in 2019 the half marathon PD detail was on EVENT 5. Chances are in 2020 they'll be on EVENT 5 again.
That's why I take notes on each event and what talkgroup(s) I see in use so know what to expect in years to come.
In my area, on the state system, while that is basically true for major events, they change almost daily for day to day events from water or hiking rescues, to local, state or federal LE activities, funeral escorts across county lines, or, now adays, protests. I've even heard them change mid-day. This is why it is very important to keep lists of the radio ids on the system, take notes, keep an Excel spreadsheet, add them to the wiki. I started that a few years ago and have almost 5000 state radios and another 5000 local/county radios identified. I began the wiki page for my state and refer to it on a regular basis. Really helps you identify not only who is on event talkgroups but also new talkgroups found or who is using encrypted talkgroups.