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You can still use rounded and flat, they just moved to the underlying VField component.
I think your v2 select would end up without any frame at all, in which case rounded won’t do anything visible. Add variant="solo" with flat to get the same effect in v3 (see playground)
As to the v-application--is-ltr class, it is called v-locale--is-ltr in V3.
If you still need it depends on your app, those are custom rules. Check if the custom mr class is used and how the margin on the first icon in lists look like (I think list icons are now set through the prepend slot, so the class would be .v-list-item__prepend .v-icon)
Source:stackexchange.com