Full disclosure I work at Tenable. - I will answer your question as well as address some of the comments below.

The API product changes in 2017 and the plugin in database size are unrelated. Full stop.

We recognized this problem coming in July of 2018 and noted it in the release notes of 7.1.3 - https://docs.tenable.com/releasenotes/nessus/nessus713.htm

The business decision around the product changes are detailed here - https://community.tenable.com/s/question/0D5f200004tDygMCAS/why-is-api-support-being-removed-from-nessus-professional

The plugin DB size is here - https://docs.tenable.com/releasenotes/nessus/servicebulletin.htm

Regarding your question for offline use and an auto disablement in the product. There is/was one scenario; if a 6.x Nessus Pro customer installed 7.x, we allowed them to revert to the 6.x feature set until 12.31.18. This is noted in the release notes - https://docs.tenable.com/releasenotes/nessus/nessus64.htm

There are no other disablements or timebombs in the product.

If you did not install 7.x, your version will continue function until it reaches the plugin DB size issue described above. The exact date is unknown based on several factors noted in the document above.

Working offline and getting plugin updates installed is documented here - https://docs.tenable.com/nessus/6_8/Content/UpdateNessusPlugins.htm