It is now. Was it then? AFAICT from https://en.wikipedia.org/wiki/PDP-11 none of the PDP-X machines had a segmented architecture.

Even so, https://sourceware.org/binutils/docs/as/PDP_002d11_002dPseud....

  9.34.2 Assembler Directives
  The PDP-11 version of as has a few machine dependent
  assembler directives.
  
  .bss
  Switch to the bss section.

  ...