Please tell us any letters associated with the Lexmark printer model so we can help you serach. I can find "X500" on the Lexmark web site but not simply a "500."
Lexmark has been historically poor at Mac OS support. The X500 printer I found shows "discontinued" and the last drive updates shown for any OS were in 2012!!
https://support.lexmark.com/en_us/drivers-downloads.html?q=x500
If you need clarification, you need to ask Lexmark. Since the dawn of personal computing, app/driver developers have been responsible for seeing that their apps keep up with new operating system developments. At least for Lexmark, it seems they chose not to support in the case of some models.

I only see seven Lexmark printer on the Gutenprint page Ronasara links; none have the number 500 in their model designators.