Plumbing vent terminated into chimney in the attic. The chimney is also used for gas-fired water heater and furnace in basement to exhaust combustion by-products to the exterior.
How/would you comment on and why?
Thanks in advance.
How do you know this? That pic alone does not tell me the entire story, so I have no opinion yet.
Not 100% sure, why do you ask?
Because you asked for opinions. I can not/will not comment without all the facts. You didn’t provide them. I asked for them.
I am trying hard to think what would be wrong with sticking a galvanized plumbing vent into a chimney and coming up blank other than it is probably not to code. (vent terminations shall be in open air), but who knows what the code was back when this was done, and maybe the chimney is open air???
Or maybe the chimney draft would affect the traps drying out quicker? (probly not much)
It might be a good solution to plumbing vents freezing in winter 
I am a building contractor and spent several years restoring historic homes in Charleston WV. I came across this in a home built in 1917 that just happened to have one of the original toilets in the basement. There were three connections. The waste of course. The water supply and an 1 1/2" connection at the back of the toilet that connected to a series of vent holes under the rim. This ran directly to the chimney. I imagine these became unpopular because any backdraft would bring CO and smoke into the home through the toilet. Interesting history though.
This is a really good question. Code obviously does not apply, however it may provide some guidance depending on interpretation. IRC with GA Amendments 2018.
I would have a difficult time marking this as a defect without some visible indicators of a problem. But an upgrade type recommendation could be made I suppose.
I had never heard of this so I had to go look up what this third connection might be for. Turns out it was an early version of a fart fan. According to the referenced source below, they may have actually worked quite well. Pretty cool!
Can you explain how this would happen?
I think he’s referring to something like below, with the side outlet being connected to a chimney. Some on the internet say this side outlet was an early form of fart fan with the draft effect in the chimney pulling any smells up and out. Others say it was used to drain a nearby sink into the toilet. What say you?
https://www.reddit.com/r/Plumbing/comments/p661o6/opening_in_my_toilet_bowl_should_it_connect_to/
I imagine it could happen if there was a fire burning in a fireplace or the furnace or hot water heater were on and a negative pressure developed inside the bathroom. This could happen from an exhaust fan, flushing the toilet or an exterior pressure change do to wind pressure or any number of environmental changes. The fumes from the chimney could be sucked into the bathroom. This is not unheard of.
I really have no idea exactly why this type of venting had stopped being used. I just know it was very short-lived and almost all these types of toilets were replaced.
Not quite what I had encountered. I wish I had taken a picture. The bowl had about an 2" opening at the top that went to a wall mounted tank and an 1 1/2" opening at the back of the bowl that was still connected to the chimney. The other toilet in the house (upstairs) was from the 40’s and no longer used that venting system though the 1 1/2" pipe was still in the wall and connected to the chimney but filled with plaster.