Block-based Widgets Editor – how to enable a title

But I see no way to add a title unless using legacy widgets.

Correct, legacy old style widgets have titles, normal blocks do not because they are not widgets.

Note that old style widgets are actually blocks of type core/legacy-widget. Everything is a block.

Has this field been removed?

No, it was never there to begin with.

It’s not that blocks can now be widgets. It’s the widgets can now be blocks. The new sidebar areas contain blocks, not widgets. Some of these blocks are a compatibility block named legacy-widget, which will render handle titles etc. You can see that here. Unlike before, sidebars are now block HTML markup, the same as posts. Legacy widgets may still refer to the old method of storing widgets but other blocks store data the same as they would elsewhere.

As for normal blocks that are not widgets, giving them each a title doesn’t make sense unless the design explicitly states that blocks are widgets, and it does not. Anybody with such a mental model of how modern widget areas work is mistaken.

If you want blocks to be surrounded by widget titles and widget markup, you should build a block for that which supports nested blocks.

In the longterm, consider widget areas a legacy backwards compatibility feature. The future is reusable blocks, or FSE templates.