The right solution to this, of course, was for me to go update a bunch of webapps to not use a deprecated URL…HOWEVER 
In case anyone else is getting ERR_INSECURE_RESPONSE errors on some of their older projects, it seems the cert has expired on spark.io. Changing it to particle.io worked great, but required digging up some code I haven’t touched in a while.
Particle folks, dunno what your end of life plan for that domain is, but I guess this could also be a heads up for you.