The migrate action (although no allowed in the frozen phase) would seem
to convert `frozen` to `data_frozen,data_cold,data_warm,data_hot` tier
configuration. As the migrate action is not allowed in the frozen phase
this would never happen, however the code is confusing as it seems like
it could.
The migrate to data tiers routing service shared the code used by the
`migrate` action that converted `frozen` to
`data_frozen,data_cold,data_warm,data_hot` if it would encounter an
index without any `_tier_preference` setting but with a custom node
attribute configured to `frozen` e.g. `include.data: frozen`
As part of https://github.com/elastic/elasticsearch/issues/84758 we have
seen frozen indices with the `data_frozen,data_cold,data_warm,data_hot`
tier preference however we could never reproduce it.
Relates to https://github.com/elastic/elasticsearch/issues/84758
f8b367d2bc