Keep in mind that for WordPress the second digit is the major version. So 4.8, 4.9, 5.0 are all equally significant updates, at least when it comes to backwards compatibility. “WordPress 4” and “WordPress 5” are not completely different versions of WordPress. 5.0 is an incremental release following 4.9.

In 5.0 nothing changed so significantly that a plugin that worked in 4.9 won’t work in 5.0. The only exception is in regards to the editor. 5.0 introduced a new editor, so if your plugin dealt with the editor, then it will likely need updates. But your plugin will continue to work if the user opts to use the old editor, or if your plugin had nothing to do with the editor.