I don't know exactly what these BSD things did, but there is a super easy way nowadays to get the stack for any process:

    eu-stack -i -p $(pidof ...)

Thanks to debuginfod this will even give you good backtraces right away (at the cost of some initial delay to load the data from the web, consecutive runs are fast). If you get a "permission denied" error, you probably need to tweak kernel.yama.ptrace_scope=0