Hi,
But where you have mentioned that your plugin will be compatible with which version? As there is no information available so we purchase it considering that it’s compatible with every version.
Every plugin gives this information that their plugin will be applicable from WordPress following version, which is not mentioned on your plugin so it will consider that there is no version restriction available.
We can not upgrade the version currently as this site is more than 5 years old and has a very stable front end and back end.
So either make it compatible with the given version or provide an alternate solution rather than upgrade the version.
Thanks