In an actual DEC VT52x terminal the operation of the <[X] (a.k.a. [Backspace]) key is controlled by a DEC private mode setting, DECBKM. This setting allows applications to control whether the key sends the DEL or the BS character. (It's also reachable via the menu on the terminal's local user interface.)
Neither the vt nor the syscons subsystems implement that. So all vt and syscons keyboard maps explicitly state either "bs" or "del" for that key.
The nosh toolset's virtual terminal subsystem does implement DECBKM, however. So it requires an extension to the kbdmap(5) format. It employs this extension in an overlay keyboard map supplied in the toolset, soft_backspace.kbd, that is applied on top of the system-supplied ones during keyboard map import.
That extension is "bspace", defined as causing the emission of BS or DEL according to the current value of the DECBKM setting.
Please don't use this name in the vt or syscons systems, unless you're extending them likewise. Thank you.