The successor to whatever desktop Lightroom you have is called Lightroom Classic, and it can only be purchased via the Adobe Photography Plan (not the confusingly named Lightroom Plan), and it is available by subscription only. If you know how to use your old version of Lightroom, then you will know how to use Lightroom Classic, although of course there are plenty of new features.
The problem is your old operating system. What operating system is it (give us the NUMBER)? The current versions of Lightroom Classic may not run on your operating system. Please let us know.
It may be (I'm not a MAC expert) that older versions of Lightroom Classic will run on your operating system — or maybe not.
What you should do is download and install the 7-day free trial to see if it will run. If it does, it will probably install an earlier version that does run on your operating system, such as Lightroom Classic 8.3, rather than the current version of 10.2 which will not run on OSX 10.13.
Of course, with such an old computer, Lightroom Classic may run very slowly, which is what I would expect. And the speed depends on your camera, if it takes high-megapixel photos (which most cameras do today), then that will slow things down further. What camera is it?
Thanks, I have also asked the questions on an apple community support page. I think it should work, but how slow, I will have to see. I bought an older nikon d7200 24 megapixles. I want to shoot raw to do some portraits and sports. I think I'll do the trial and see how it goes.