Would anyone know whether it's possible to get Old Value and New Value audit on transaction lines specifically for tax code or tax account via n/query or any other way really?

You can do a Transaction search and join Line System Notes

I've tried but Line system notes doesn't present my field of interest like tax item

even if it is not exposed on UI, you can try in suitescript with the script id of the field, it may return results.

thanks Selcuk, it doesn't seem immediately possible true