Nelio A/B Testing adds the nab query arg to the URL of your tested pages. This query arg tells your WordPress server which variant should be loaded for the current visitor and, therefore, they should only be visible when there are tests running.
If you’re still seeing this query arg in (some of) your URLs after you stopped all tests and/or after deactivating Nelio A/B Testing completely, it’s probably because your website is using a cache plugin and/or behind a CDN. Simply purge/clean these systems and the query argument should be gone.