For land ownership records and other similar scenarios such as business directors, the requirement for these to be public will be in legislation rather than a contract - this provides the legal basis, see GDPR Article 6(1c). Additionally when government departments are doing it they also have 6(1e) as lawful basis:
"1. Processing shall be lawful only if and to the extent that at least one of the following applies: ...
(c) processing is necessary for compliance with a legal obligation to which the controller is subject; ... (e) processing is necessary for the performance of a task carried out in the public interest or in the exercise of official authority vested in the controller;" -- GDPR, Article 6(1c,e).
I'm not familiar enough with the specific legislation that will apply here but pretty sure this will be the case, and having said this you may well find public registries also become less public going forward.
The reason ICANN has come under fire, is partly because under GDPR privacy is a protected fundamental right and therefore to comply personal data should be kept private by default and privacy never something you would be required to pay extra for. Any contract ICANN have in place with their registrars will not override legislation, it is in fact the other way around.
"2. The controller shall implement appropriate technical and organisational measures for ensuring that, by default, only personal data which are necessary for each specific purpose of the processing are processed. That obligation applies to the amount of personal data collected, the extent of their processing, the period of their storage and their accessibility. In particular, such measures shall ensure that by default personal data are not made accessible without the individual's intervention to an indefinite number of natural persons." -- GDPR, Article 25(2).
This doesn't stop ICANN from maintaining a register of domain name owners (registrants), but it does mean they can't just publish all records upon request to anyone anymore - whether people will be granted access to personal data will now depend on if they have a lawful basis for this, and in these cases their processing of the personal data will be limited to those purposes. Being nosey doesn't count!
"When assessing whether consent is freely given, utmost account shall be taken of whether, inter alia, the performance of a contract, including the provision of a service, is conditional on consent to the processing of personal data that is not necessary for the performance of the contract." -- GDPR, Article 7 (4) - Conditions for consent.
What this means essentially, is that if the consent is conditional for the contract it will not be treated as freely given, and therefore not valid - it will no longer be acceptable to contractually bind the provision of a product or service with consent to publish personal data or any other form of processing such as marketing mailing lists.
Looking now at the specific points you have raised:
"GDPR article 6 allows for processing of personal data on a contractual basis (section 1b)"
Whilst this is true, this is only part of it - it doesn't allow for unlimited processing for any purpose and sharing it with any people, if you look at Article 5(b) it states that the information is collected for specific explicit legitimate purposes. Each purpose requires its own legal basis and needs to be compatible with the principles of GDPR. Without consent, ICANN currently does not have a legal basis to make the WHOIS records public for EU citizens and should have adopted some technical controls to require them to opt-in if they wish to be included in the public register. They're coming under fire for non-compliance having been given 2 years to prepare and change their systems/processes.
"processing is necessary for the performance of a contract to which the data subject is party or in order to take steps at the request of the data subject prior to entering into a contract;"
Again whilst this is true, not all processing is necessary for the performance of a contract. In the same way people must give consent to receive marketing communications, they must freely give consent for their information to be shared/published (separate to the contract for provision of service) in the absence of other lawful basis for this processing.
"Also section 1c, processing is necessary for compliance with a legal obligation to which the controller is subject;"
There is no legislation which requires them to publish the personal data of domain name registrants. In this paragraph 'legal obligations' refers to those required by legislation (i.e. statutory obligations), not contracts (or non-statutory obligations) which are covered under Article 6(1b).
"And finally section 1e, processing is necessary for the performance of a task carried out in the public interest or in the exercise of official authority vested in the controller;"
ICANN has no official government-assigned authority, and publishing the personal data globally is not in the interests of the data subject's or others' welfare or well-being - this is what is meant by 'public interest'.
As far as I can see what ICANN are actually doing to comply seems to be accepted by the European Data Protection Board, they are only 'under fire' as you say because they are late in doing so. The deadline was 25th May 2018 and they had 2 years to prepare like all other organisations.