"Cannot add field", row size "is greater than maximum allowed size (8126) for a record on index leaf page"

CiviCRM 5.21.1; Spotted the following in MariaDB logs:

[Warning] InnoDB: Cannot add field `postal_greeting_display` in table `wordpress-civicrm`.`civicrm_contact` because after adding it, the row size is 8486 which is greater than maximum allowed size (8126) for a record on index leaf page.
[Warning] InnoDB: Cannot add field `confirm_text` in table `wordpress-civicrm`.`civicrm_event` because after adding it, the row size is 8275 which is greater than maximum allowed size (8126) for a record on index leaf page.

No idea how serious this is... Any ideas how to address the issue? Thanks.