The list that you pass with this action completely replaces the existing list of thumbprints. (The lists are not merged.)
Typically, you need to update a thumbprint only when the identity provider's certificate changes, which occurs rarely. However, if the provider's certificate does change, any attempt to assume an IAM role that specifies the OIDC provider as a principal fails until the certificate thumbprint is updated.
Because trust for the OIDC provider is ultimately derived from the provider's certificate and is validated by the thumbprint, it is a best practice to limit access to the UpdateOpenIDConnectProviderThumbprint
action to highly-privileged users.