just visited the this URL for office add-in availability.
What is one time purchase in office add-ins and office 365 subscription ones?
Office 2019 is sold as a one-time purchase, which means you pay a single, up-front cost to get Office apps for one computer. One-time purchases are available for both PCs and Macs. However, there are no upgrade options which means if you plan to upgrade to the next major release, you'll have to buy it at full price.
Office 365 is a subscription service that ensures you always have the most up-to-date modern productivity tools from Microsoft. There are Office 365 plans for home and personal use, as well as for small and midsized businesses, large enterprises, schools, and non-profits.
A detail difference can be found at https://support.office.com/en-us/article/what-s-the-difference-between-office-365-and-office-2019-ed447ebf-6060-46f9-9e90-a239bd27eb96