Hi, good question.
Assuming you have kubecost addon deployed in your eks 1.29 cluster, once you upgrade to 1.30 then kubecost will remain and it will be working regardless whether it is officially supported by eks addon.
In case you decide to do a new install after eks upgrade then I would expect external addons to be a bit behind with support of latest eks version and so you may not be able to install it at this time.