You’ll want to see here. Tabinine is run by a company and so you should contact them. Jupyter Notebook 7+ is built on JupyterLab components and so in general, extensions current with it will work. Tabnine hasn’t been updated in three years according to the repo and issues there. There are more current resources of this type, for example here.
You may be able to keep your current extensions working with NbClassic. See here for more about that.
There is an extensions manager built in to Jupyter Notebook 7+. From the top menu choose ‘View’ > ‘Extension Manager’. Note there is a ‘DISCOVER’ section. This may give you another starting point for a list of available extensions. Keep in mind that JupyterLab may offer more full-featured extensions because it is more flexible as the emphasis for Jupyter Notebook 7+ is to be document-centric.