600MHz equipment will still work. It will just be working in a frequency band that is deemed illegal for non-licensed operation (save for a few "guard bands", which are narrow bands in that frequency spectrum that can be used by equipment made to be used ONLY in those guard bands). Using equipment in that 600MHz space runs the risk of interference, both FROM and TO any 5G devices working in that bandwidth. The potential exists for the illegal user to be caught and face hefty fines.
400MHz, 500MHz and 900MHz are the frequency ranges I've been seeing used mostly in professional settings. Generally, the more expensive units from Shure (Axient, ULX, QLX) and Sennheiser (Digital 9000, 6000, 5000, 3000) do allow for more units to be used in those frequencies, while the less expensive units from those respective companies generally don't use as narrow a bandwidth, thus limiting the amount of units that can be used in the same space.
2.4gHz units can't be run together in large quantities - from what I remember the Sennheiser 2.4gHz units recommend only a total of 4 units run at a time. The aren't reliable at a distance.
I'm surprised at this question being asked now, as this is basically old news. Did the switch over in the 600MHz band just happen in your area? It happened a year ago in my area.