Will an external USB floppy drive work with a modern Mac?
The short answer is yes, external USB floppy drives will still work with macOS. 

However, the issues you're likely to face relate to:


how the actual floppy disks are formatted (both in terms of file systems and capacity)
the type of floppy disks involved (i.e. are we talking 1.4MB disks, or older 800KB or even 400KB floppies)
the condition of the floppy disks (specifically, are they now demagnetised?)


Assuming they're 1.4MB floppies then you should be able to read the data. You won't, however, be able to write data to the floppies (although in your case this shouldn't be an issue as you're wanting to copy the data from them, not to them).

If they're 400KB or 800KB floppies, you can pretty much forget about using Mac OS X / macOS.

If they're demagnetised, you can pretty much forget about using anything to read the data.