We use an older AKG wireless mic system for a campground pastor we support. Probably 25 years old or so. It's in the old 600 range but we've never had interference and no complaints. I'm thinking this could be due to most of the campgrounds being somewhat remote so there are no dedicated users of the bands we are on in those areas. But, one of those campgrounds is Palm Springs Thousand Trails (actually in Palm Desert CA) which is very close to a lot of city, industry, emergency services, and even an airport. Still no issues. Maybe it simply doesn't transmit or receive from/to much distance at all. I also have a quite old small AKG guitar BUG and used it in the YMCA building close to the Palm Desert Police station every Sunday morning for around 2 years or so with no issues. But, I know for fact the guitar bug only reaches around 20 - 30 feet in line of site. Stage to pedal board is about it's limit. Plus it would devour one AAA battery every Sunday morning.
There is a way to find the frequencies in use in your area and who they are licensed to on the FCC website. If your frequencies are not in use then use your older stuff at your own discretion.

Does the noise in your head bother me ?