In Vivado 2015.x, the write_debug_probes command is called as part of the opt_design step in the project flow.
However, opt_design is an optional step. It can be disabled for example, by running the following:
set_property STEPS.OPT_DESIGN.IS_ENABLED 0 [get_runs impl_1]
This means that Vivado will not generate the .ltx file when opt_design is disabled.
In Vivado 2016.1 the "write_debug_probes" call is moved from the opt_design step to the init_design step.