@Thrrreeee- This is correct - the attribute is currently not recognized by tooling.
Plans currently include to add support for this attribute in gSYM tooling - see related RFC: Extending gSYM Format with Call Site Information for Merged Function Disambiguation
There are no immediate plans to add support to LLDB or GDB. Is this causing any issues with the debuggers ? This attribute is gated by a usage flag (-emit-func-debug-line-table-offsets) so I was hoping to avoid causing widespread issues in various tooling.