In general users should not be affected by an update to NatTable 2.1. We introduced some special public constants that were needed to fix persistence issues, which should not be an issue. Apart from that there are some new methods.
Only the update to Nebula 3.0 might be problematic for users that still use Java 8 for their products, as Nebula decided to increase the minimum required Java version to Java 11. The NatTable Nebula Extension actually haven't updated the minimum required Java version and also the lower bound of the package imports are not changed. This way it is still possible to use the Nebula Extension with an older version of Nebula. Only the NatTable Nebula Extension Feature that can be installed via p2 will contain the Nebula 3.0 artefacts that require Java 11.