I need to purchase a new iMac , my old one is a late 2009 and I can not update the software . Will the new iMac 's have a SD card slot?

no, but you can get an usb-c to sdcard adaptor pretty cheap like this

Amazon.com: SD Card Reader, uni Sturdy USB-C to Micro SD Memory Card Reader Adapter ( 2TB Capacity, 5Gbps Transfer) Thunderbolt 3 Compatible with Android Galaxy S20, MacBook Pro/MacBook Air and More : Electronics

think the only macs these days with an sd card port is

MacBook Pro 14” and 16” models & Mac Studio

A while back I purchased one of the external SD units. It works great. And I can easily connect it to any of our other computers if needed by just plugging it into one of the USB ports.