I asked on Reddit and no answers:
https://www.reddit.com/r/kustom/comments/flch10/is_there_any_way_to_check_update_frequency_setting/
So I assume this is impossible and I'm posting this as an idea.
Examples of better usability:
- Clock widget with seconds that make sense to show only in fast mode with text like $if(st(updatemode) = FAST, :df(ss))$
- Countdown widget that increases accuracy for last minutes with a similar if condition.