> Do we need separate jobs and ABI dumps for each platform and arch? I guess we need at least separate dumps for 32 and 64bit.

Not really, check what happened in my 64 build system when I did the changes that broke the ABI in the latest 3.9 release:

7a3947dec3d8">7a3947dec3d8">root@7a3947dec3d8:/pytho# abidiff Python-3.9.2/python Python-3.9.3/python
Functions changes summary: 0 Removed, 3 Changed (53 filtered out), 0 Added functions
Variables changes summary: 0 Removed, 0 Changed (1 filtered out), 0 Added variable

3 functions with some indirect sub-type change:

  [C]'function void PyEval_AcquireThread(PyThreadState*)' at ceval.c:381:1 has some indirect sub-type changes:
    parameter 1 of type 'PyThreadState*' has sub-type changes:
      in pointed to type 'typedef PyThreadState' at pystate.h:20:1:
        underlying type 'struct _ts' at pystate.h:51:1 changed:
          type size hasn't changed
          4 data member changes (2 filtered):
           'char _ts::recursion_critical' offset changed from 296 to 320 (in bits) (by +24 bits)
           'int _ts::stackcheck_counter' offset changed from 320 to 352 (in bits) (by +32 bits)
           'int _ts::tracing' offset changed from 352 to 384 (in bits) (by +32 bits)
           'int _ts::use_tracing' offset changed from 384 to 416 (in bits) (by +32 bits)
          1 data member change:
           type of 'char _ts::overflowed' changed:
             type name changed from 'char' to 'int'
             type size changed from 8 to 32 (in bits)
           and name of '_ts::overflowed' changed to '_ts::recursion_headroom' at pystate.h:61:1

  [C]'function int _PyErr_CheckSignalsTstate(PyThreadState*)' at signalmodule.c:1684:1 has some indirect sub-type changes:
    parameter 1 of type 'PyThreadState*' has sub-type changes:
      in pointed to type 'typedef PyThreadState' at pystate.h:20:1:
        underlying type 'struct _ts' at pystate.h:51:1 changed:
          type size hasn't changed
          no data member changes (6 filtered);
          no data member change (1 filtered);

  [C]'function void _PyErr_Clear(PyThreadState*)' at errors.c:426:1 has some indirect sub-type changes:
    parameter 1 of type 'PyThreadState*' has sub-type changes:
      in pointed to type 'typedef PyThreadState' at pystate.h:20:1:
        underlying type 'struct _ts' at pystate.h:51:1 changed:
          type size hasn't changed
          no data member changes (6 filtered);
          no data member change (1 filtered);