A mere decompilation or general reverse engineering should be fine in many if not most jurisdictions [1]. But it is a whole different matter to make use of any results from doing so.

[1] https://www.law.cornell.edu/wex/reverse_engineering

Using an LLM (or any technique) to decompile proprietary code is not clean room design. Declaring the results "open source" is deception and theft, which undermines the free open source software movement.

Only if you use the decompiled code. But if one team uses decompiled code to write up a spec, then another team writes an implementation based on that spec, then that could be considered clean room design. In this case, the decompiler would merely be a tool for reverse engineering.

It is true that at least some jurisdictions do also explicitly allow for reverse engineering to achieve interoperability, but I don't know if such provision is widespread.