The column abuse_filter_log.afl_log_id was introduced in 2012 with rEABF5e4289ce4e240f5a91e7c6fd68bab32b8fe7074c, and is unused since then. It's unclear how it should be used, and there's no plan to ever use it.
The patch to drop the field has already been merged on master with https://gerrit.wikimedia.org/r/#/c/mediawiki/extensions/AbuseFilter/+/499770/.
# **ALTERs to run:** https://gerrit.wikimedia.org/r/#/c/mediawiki/extensions/AbuseFilter/+/499770/2/db_patches/patch-drop_afl_log_id.sql
# **Where to run those changes:** all.dblist
# **When to run those changes:** As soon as wmf.11 reaches production. Since the column is unused there's nothing to be worried about I guess.
# **If the schema change is backwards compatible:** Yes, as long as all uses of the column will be removed in wmf.11
# **If the schema change has been tested already on some of the test/beta wikis:** Just a bit locally