In the development version of bsst (and upcoming version 0.1.2) plugin system was reworked, and the way to run the referenced plugin slightly changed. Now it has to be run with ./bsst-cli --plugins=plugins/op_checkcontractverify_bsst_plugin.py. The CAT opcode can now be enabled also via --explicitly-enabled-opcodes=CAT
The github gist with the plugin was also updated to work with new version.
(I would prefer to edit the first post to update the info, but it seems that I cannot)
(edit 29-11-23: updated the gist again)