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Try radare2. There is also GUI fronted called cutter (https://cutter.re/).

Evan’s debugger gets you most of the way there: https://github.com/eteran/edb-debugger

You have to build from source but it’s easy to do.

I’m thinking of devoting a summer to writing an Ollydbg-like UI wrapper for LLDB, because I miss Olly so bad when I leave Windows.

Not sure if I understood what genre you have in mind. Does Ghidra fit the bill?

I meant a debugger that doesn't require or expect you to have the source code of the application you're debugging and just shows you disassembly, lets you use instruction execution/memory access breakpoints, patch instructions, etc.

Ghidra doesn't have a debugger yet, but it's a planned feature.

Ghidra doesn't have a debugger (which afaik is a WIP).