As the Fleet documentation mentions some integrations are installed (and maybe managed) by Fleet and some or all of them have an option in the 'Settings' tab of the integration to Keep integration up to date automatically. Other integrations do not have this capability and would be updated manually when a new version is available.
https://www.elastic.co/guide/en/fleet/current/upgrade-integration.html#upgrade-integration-policies-automatically

Terraform providers offers an integration installation capability where an option for 'version' specifies a specific version. This option is a type string.
https://registry.terraform.io/providers/elastic/elasticstack/latest/docs/resources/fleet_integration

Questions are: 1) Could an value of "latest" or some other string be used in place of a specific numbered version to 'enable' terraform to Keep the integration(s) up to date? 2) Could the Fleet API with 'keepPoliciesUpToDate' option be used to set/enable all integrations across the board to be kept update to date automatically? 3) Is it advisable to try this or would there be any known or unintended consequences that might occur thereby making it something not to do or attempt?

It seems that since some of the integrations have the keep up to date feature and some do not, that there is a reason that not all integrations already have this Fleet UI toggle button.