What client for DECwindows

This is supported only by some Linux filesystems:

chattr +s sample.txt

may (or may not) do what you want.

From man chattr:

NAME
       chattr - change file attributes on a Linux second extended file system
...
       When a file with the ‘s’ attribute set is deleted, its blocks are
       zeroed and written back to the disk.  Note: please make sure to read
       the bugs and limitations section at the end of this document.
...
BUGS AND LIMITATIONS
       The  ‘c’, ’s’, and ‘u’ attributes are not honored by the ext2
       and ext3 filesystems as implemented in the current mainline Linux
       kernels. These attributes may be implemented in future versions of
       the ext2 and ext3 filesystems.

I do not know which specific mainline kernel versions (if any) implement this.

I sincerely doubt there's an exact analog. The nature of a Linux process (and consequently of running a command) is so different than the nature of a VMS process and how it runs a command that nothing like this can really exist.

As I understand VMS, when you run a command like TYPE SOMEFILE.TXT, DCL reads the TYPE.EXE into the memory of the same process that DCL runs in, fixes up all the relocations that TYPE.EXE needs, and then jumps to the first instruction of TYPE.EXE. When TYPE.EXE has done its work, the OS returns control to the DCL code that still resides in the same process as TYPE.EXE.

That heavyweight process, which includes DCL, a table of lexicals, and a well-known jump table of addresses of various things, allows Ctrl-T to work. That key sequence is just a special form of keyboard interrupt, and the DCL code in any given process can do the work you describe.

A linux process doesn't always have a shell (DCL analog) in it. A linux process may not even have keyboard interrupt handler code registered with the kernel, which would mean that any keyboard interrupt (Ctrl-C or Ctrl-) gets the kernel to cause the process to exit.