There’s no extra gate or dedicated staff member in my store only whoever’s at the till and if the self-checkout is busy they’re too busy to watch them.
What I did notice though is that they now put anti-theft tags on more stuff, e.g. the ones on big packages of sausages are new. But it’s still the same open beep gate at the end, which I actually triggered exactly once and that was when using the manned checkout, they’re older and cashiers need to deactivate the tags manually (and they missed my coffee), the self-checkout ones apparently do it reliably when you’re scanning the item.
Over time I think that’s probably where this is heading. The store still uses those very old EM fuses/amplifiers as anti-theft tags and of course ordinary barcodes, at some point the larger industry is going to switch to RFID for everything and every item will know whether it’s been paid for.