https://we.phorge.it/book/phorge/article/custom_fields/ describes that setting as

• search: Show this field on the application's search interface, allowing users to filter objects by the field value.
• fulltext: Index the text in this field as part of the object's global full-text index. This allows users to find the object by searching for the field's contents using global search.

(emphasis by mine)

We're basing cluttering the global fulltext search index with some useless stuff that you would not ever want to search for globally.

Fields currently getting indexed per https://phabricator.wikimedia.org/config/edit/maniphest.custom-field-definitions/ to fix:

• "release.version": { "name": "Release Version",
  • Change "fulltext": true to "fulltext": false
  • Potentially delete DB rows: DELETE FROM phabricator_maniphest.maniphest_task_ffield WHERE fieldKey = "!jdY";

• "release.date": { "name": "Release Date", (last indexed 1537747200 (2018-09-24 00:00:00))
  • Potentially delete DB rows: DELETE FROM phabricator_maniphest.maniphest_task_ffield WHERE fieldKey = "!kmK";

• "train.backup": { "name": "Other Assignee",
  • Change "fulltext": true to "fulltext": false
  • N/A - Potentially delete DB rows: None as indexing had always crashed, see T398197: 2100 tasks cannot be found by fulltext search (due to exception when indexing "Other Assignee" custom field)

• "train.status": { "name": "Train Status",
  • Change "fulltext": true to "fulltext": false
  • Potentially delete DB rows: DELETE FROM phabricator_maniphest.maniphest_task_ffield WHERE fieldKey = "!WBV";

• "deadline.due": { "name": "Due Date",
  • Change "fulltext": true to "fulltext": false
  • Potentially delete DB rows: DELETE FROM phabricator_maniphest.maniphest_task_ffield WHERE fieldKey = "!GGo";

• "risk.summary": { "name": "Summary", (is this ever queried by Sec?)
  • Change "fulltext": true to "fulltext": false
  • Potentially delete DB rows: DELETE FROM phabricator_maniphest.maniphest_task_ffield WHERE fieldKey = "!H_R";

• "risk.impacted": { "name": "Impacted", (is this ever queried by Sec?)
  • Change "fulltext": true to "fulltext": false
  • Potentially delete DB rows: DELETE FROM phabricator_maniphest.maniphest_task_ffield WHERE fieldKey = "!2xi";

• "error.url": { "name": "Request URL",
  • Add "type": "text"
  • Change "fulltext": true to "fulltext": false
  • Potentially delete DB rows: DELETE FROM phabricator_maniphest.maniphest_task_ffield WHERE fieldKey = "!jAE";

• "error.id": { "name": "Phatality ID",
  • See T398397: Remove remnants of Phatality ID in Phabricator ("error.id" custom field)
  • Add "type": "text"
  • Change "fulltext": true to "fulltext": false
  • Potentially delete DB rows: DELETE FROM phabricator_maniphest.maniphest_task_ffield WHERE fieldKey = "!Qxw";

• "deadline.start": { "name": "Est. Start Date",
  • Change "fulltext": true to "fulltext": false
  • Potentially delete DB rows: DELETE FROM phabricator_maniphest.maniphest_task_ffield WHERE fieldKey = "!AnD"; -- N/A as there are no DB rows