this shouldn't have happened, but yes, i can see that it has.
taskcluster windows instances, are supposed to be pinned to a specific occ sha revision, so that they are not susceptible to change on the master branch of the occ infra codebase.
the windows 7 ec2 amis in production were deployed on 2018-11-13.
however, the change which introduced sha pinning was deployed on 2018-11-25, after the last windows 7 deployment.
this means that the currently deployed windows 7 amis, still take their configuration from the master branch of occ and as such they will change whenever there are commits to occ master. this can happen frequently and did in fact happen on tuesday when this regression was noted. this was an oversight on my part as i hadn't noted that windows 7 prod amis predated the sha pinning change.
we will need to redeploy windows 7 (and 2012) amis in order to get them to pick up the sha pinning change from late november, to get them pinned to a deterministic occ sha revision to prevent this happening again.
once all of our windows infra is using the new sha pinned occ configuration, it will be a lot easier to say with certainty whether specific windows worker types have changed or not and point to the specific changes in occ commit history (which uses a commit message convention that points to a bug number). as of today, this is only true of windows 10 ec2 instances. we can easily include windows 7 & 2012 ec2 instances as soon as we redeploy those worker types and there is ongoing work to include windows 10 hardware instances over the coming days.