We added version 1 reference catalogs in Spring 2019 (
DM-17029), with flux fields having explicit units of nJy. Version 0 (aka unversioned files) had fluxes either in Jy or without units but assumed to be Jy. We have issued log warnings about v0 refcats that included information about how to convert them since DM-17029, so anyone trying to use them has been warned that they are old for a while. The log message did not explicitly say that format was deprecated.
I propose we end support for version 0 refcats, switching to raising an exception if the refcat loader encounters v0 files. This will remove a bunch of code from meas_algorithms and simplify the refloader a bit. I'd argue that we don't need a deprecation period for this, given that the "old style units" log warnings were in place since 2019, and there hasn't been code to generate v0 refcats since that time, but we could also change the v0 warnings to include a deprecation note if required.
I believe all our supported v0 refcats have already been converted to v1 (compare the v0 and v1 subdirectories of /datasets/refcats/html on lsst-devl), and all of our well-documented refcat generation/conversion code was written after the above date, so it's pretty likely no user-generated v0 refcats exist in the wild.