Yeah, it's currently broken. Unless you're running iOS 12 and Safari 12 on macOS 10.13 or 10.14 it won't work at all. Some reports seem to suggest that running iOS 13 and Safari 13 on macOS 10.14 might work, others indicate that it's working somewhat with iOS 13 and macOS 10.15 but in all other cases you're screwed.
OK, I just restarted the iMac (running 10.4.6) and my iPhone tabs came back.. But if I look at my iPad (running 12.4.1) the tabs are perfect - My iPhone (running iOS13) I've no tabs.... I'm afraid to update to iPad OS today in case I lose the tabs on iPad (which is the only device still working perfectly...see attached of iPad and iPhone