  We present Spitzer follow-up imaging of 95 candidate extremely cold brown
dwarfs discovered by the Backyard Worlds: Planet 9 citizen science project,
which uses visually perceived motion in multi-epoch WISE images to identify
previously unrecognized substellar neighbors to the Sun. We measure Spitzer
[3.6]-[4.5] color to phototype our brown dwarf candidates, with an emphasis on
pinpointing the coldest and closest Y dwarfs within our sample. The combination
of WISE and Spitzer astrometry provides quantitative confirmation of the
transverse motion of 75 of our discoveries. Nine of our motion-confirmed
objects have best-fit linear motions larger than 1"/yr; our fastest-moving
discovery is WISEA J155349.96+693355.2 (total motion ~2.15"/yr), a possible T
type subdwarf. We also report a newly discovered wide-separation (~400 AU) T8
comoving companion to the white dwarf LSPM J0055+5948 (the fourth such system
to be found), plus a candidate late T companion to the white dwarf LSR
J0002+6357 at 5.5' projected separation (~8,700 AU if associated). Among our
motion-confirmed targets, five have Spitzer colors most consistent with
spectral type Y. Four of these five have exceptionally red Spitzer colors
suggesting types of Y1 or later, adding considerably to the small sample of
known objects in this especially valuable low-temperature regime. Our Y dwarf
candidates begin bridging the gap between the bulk of the Y dwarf population
and the coldest known brown dwarf.
