Which installation parts are backward compatible?

I've been examining multi-major-version binary packaging again, and I was
wondering whether we have a good overview over which pieces of the
installation are backward compatible (that is, they can be shared between all
major versions) and which are not. For example, psql 8.4 can now presumably
be shared between all major versions, so previous schemes to have several
psqls installed can be simpified.

Here is a start:

Item Compatible across major versions?
(i.e. the 8.4 version works with 7.4+ server)

clusterdb yes?
createdb yes?
createlang yes?
createuser yes?
dropdb yes?
droplang yes?
dropuser yes?
ecpg no?
initdb NO
pg_config (should be kept around for each major version)
pg_controldata NO
pg_ctl ?
pg_dump read yes, write NO
pg_dumpall read yes, write NO
pg_resetxlog NO
pg_restore yes?
pgtcl_*mod YES
postgres NO
psql YES
reindexdb yes?
vacuumdb yes?

If no such list exists yet, perhaps we can complete the above one, document
it, and pass it on to the packagers.