Keep in mind that your license expiration date has nothing to do with whether 3CX supports your version you are on. You must upgrade to V20 before the first of the year to ensure your PBX is supported.
But this is not just about whether your PBX is supported, you also have to consider what happens if some of the features you depend upon stop working or you are no longer compatible with registration servers. The bottom line is your drop-dead date is the end of December 2024.