Unfortunately as with all things to do with the law until a case such as the one you are describing is tested in court there is no way to know which way it will go. Common sense states that if it is impossible to do something then you can't be punished for not doing it, however the law is rarely about common sense and more often than not is what a previous court has ruled should have happened to begin with (precedent).
The GDPR is very dangerous and should be something that all webmasters keep in their minds as it covers not just sites hosted in the EU but sites belonging to EU entities (business etc) which are hosted in other countries, websites owned by foreign entities which are hosted in the EU, as well as websites which can be accessed by EU citizens but which are hosted overseas and belong to businesses with no affiliation to the EU. Basically the only site that can't be affected by the GDPR is a website which blocks access to EU citizens and prevents them from uploading personal information and which does not collect any data or analytics on EU citizens.
This is one of those questions where it is most definitely worth while consulting a legal professional as they will be able to advise you the best on your course of action.
As a side note wordpress.com is such a large host with such a diverse and extensive customer base that it is highly unlikely that they have not run into this sort of situation before so it may also be worth while contacting Wordpress.com and asking them what they can suggest with regards to allowing your sites end users to be able to delete their personally identifyiable information. Wordpress also provides an information page on their support site for complying with the GDPR using Wordpress.com (https://en.support.wordpress.com/your-site-and-the-gdpr/)