I noticed an inconsistency between the documentation (https://docs.opnsense.org/manual/interfaces_settings.html#allow-ipv6 (https://docs.opnsense.org/manual/interfaces_settings.html#allow-ipv6)) and the current GUI. In OPNsense 26.7, the checkbox now reads "Turn off IPv6" instead of "Allow IPv6," which flips the function of the checkbox. I'm not sure in which version this change was made; it could have been before 26.7.
Suggestion: Update the documentation to reflect the new wording.