This has no practical impact on users since frozen indices are the only
throttled indices today. However this has an impact on upcoming features
that would use search throttling.
Filtering out throttled indices made sense a couple years ago, but as
we're now improving support for slow requests with `_async_search` and
exploring ways to reduce storage costs, this feature has most likely
become a trap, that we'd like to not have with upcoming features that
would use search throttling.
Relates #54058
58c3bb5ae1