No index mapper found for field: [_type] returning default postings format

Hello,
I 've been running a single node OpenSearch cluster for about 8 months on Debian 11. Upgraded from 1.2.x to 1.3.x. Today I upgraded from 1.3.5 to 2.2.1, after which I saw the following warning:

[2022-09-14T11:58:18,783][WARN ][o.o.i.s.IndexShard ] [.opendistro-ism-config][0] no index mapper found for field: [_type] returning default postings format
[2022-09-14T11:57:17,617][WARN ][o.o.i.s.IndexShard ] [.opendistro-ism-config][0] no index mapper found for field: [_type] returning default postings format
[2022-09-14T11:56:33,449][WARN ][o.o.i.s.IndexShard ] [.opendistro-ism-config][0] no index mapper found for field: [_type] returning default postings format
[2022-09-14T11:55:53,210][WARN ][o.o.i.s.IndexShard ] [.opendistro-ism-config][0] no index mapper found for field: [_type] returning default postings format
[2022-09-14T11:52:30,866][WARN ][o.o.i.s.IndexShard ] [.kibana_1][0] no index mapper found for field: [_type] returning default postings format
[2022-09-14T11:52:17,856][WARN ][o.o.i.s.IndexShard ] [.opendistro-ism-config][0] no index mapper found for field: [_type] returning default postings format
[2022-09-14T11:49:44,984][WARN ][o.o.i.s.IndexShard ] [.opendistro-ism-config][0] no index mapper found for field: [_type] returning default postings format

If I remember correctly from ElasticSearch, the _type was to be deprecated, hence I made sure my index templates never used it.

I haven’t seen this warning before and haven’t seen it anymore. Is it something that should concern me or is it something that got migrated after the upgrade?