When trying to upgrade an index with custom similarity settings from 7.1.0, the node fails to start with an exception similar to "Caused by: java.lang.IllegalArgumentException: Unknown settings for similarity of type [BM25]: [normalization.h2.c, normalization, after_effect, basic_model]" . We observed this happening upgrading from 7.1.0 to 7.5.0 and also from 7.1.0 to 7.1.1 . This appears to be the same issue as #25350.

To reproduce, start up a 7.1.0 cluster, and apply the following settings:

PUT /i
{
  "settings": {
    "index": {
      "similarity": {
        "my_similarity": {
          "after_effect": "l",
          "basic_model": "g",
          "normalization": "h2",
          "normalization.h2.c": "3.0",
          "type": "DFR"
        }
      }
    }
  }
}

Then attempt to upgrade that cluster. (Thank you @DaveCTurner for concise repro steps)