What about that legacy system? How can I know which program is using it?

A legacy system is outdated computing hardware or software that is still in use. To know which programs are using it, click the apple icon on top left most of your screen. From the drop down menu, select the About This Mac, on the Overview tab, click System Report at the bottom. There you will find information on Hardware, Network and Software. Under Software, click Legacy Software. If it says No Information found, then there are no program/s in your Mac that use Legacy Software. Usually these are programs that run in 32 bit, like Canon (Camera) EOS Utilities of old.

Additionally, some legacy extensions are used by multiple vendors. Apple is just warning us that they exist . Most vendors will be updating their apps to comply with the new operating system when it's released. There is nothing we need to do at this time. If you attempt to remove any of these extensions, you will disable your software from functioning.