A fresh install of ELK 7.10 fills the deprecation log once Kibana is installed.
[2020-12-29T14:12:46,624][DEPRECATION][o.e.d.a.a.i.a.g.TransportGetAliasesAction] [app01] this request accesses system indices: [.kibana_1], but in a future major version, direct access to system indices will be prevented by default
[2020-12-29T14:12:46,977][DEPRECATION][o.e.d.c.m.IndexNameExpressionResolver] [app01] this request accesses system indices: [.kibana_1], but in a future major version, direct access to system indices will be prevented by default
In the JSON log they are de-duplicated, but in the plaintext log they generate several per second.