Tower shields do not drop as legendary anymore but kiteshields do. If youre willing to use umbral shards then 590 tower shield is equally as good then 600 is. Kite shields do drop legendary and they can get fortify on crit and such so sometimes depending on build it might outshine tower shield even with heavy gear
I saw a 600 GS round shield on our auction with 4 perks last week. They exist but really rare since they can’t be crafted that way, has to be a drop. It’s a shame they can’t be crafted legendary, maybe one day.