The first two points under article 17 says that the data should be erased unless,
1. the personal data are no longer necessary in relation to the purposes for which they were collected or otherwise processed;
That casts a pretty wide net for the WT to use. I agree with other posters that this is what the WT will likely use to justify maintaining historical data on current and former members forever and I suspect the courts will be hesitant to challenge them on that.
To me, the most impactful element of GDPR will be the obligation to disclose, and obtain written consent initially and then periodically thereafter.