AT&T Fiber connection slowing down every night

The result is the same though. Even if they're not intentionally lowering the bandwidth, they're intentionally not doing anything about it. It's like an airline selling more seats than they have available because they're hoping some people cancel. When that doesn't happen, they end up bumping people off flights. 

This is a case of them caring more about maximizing their profit than the satisfaction of their customers. Anyone who pays for 1 gig service and only gets a fraction of it is a sucker. So, yes, it's intentional and deceptive.

I disagree only with the use of the word "throttling."  In your Airline analogy, "throttling" would mean that they leave seats empty and still bump people off the flight.

They have covered themselves nicely in the terms of service, such that factors such as peering issues are outside the scope of the provided rate:

6.2.1.4 Interconnection:

Because the internet consists of multiple interconnected networks and most internet end points (e.g., websites and other content providers) are not directly connected to AT&T's network, AT&T must connect to and exchange traffic with other networks to provide its subscribers the capability of uploading data to or downloading data from internet end points that are connected to those networks. To that end, AT&T has entered into commercially negotiated agreements to exchange traffic with such networks on mutually agreeable terms. Consistent with its longstanding practice, AT&T does not warrant that it will establish or expand the connections between its network and other networks except on such mutually agreeable terms. To the extent AT&T is unable to reach agreement on terms of interconnection or network expansion with these other networks it could affect your service. These impacts on your service performance are described in more detail in AT&T’s Open Internet notice. AT&T therefore makes no promise express or implied that you will be able to upload data to or download data from internet end points connected to other networks at any particular speed.

Like the other networks that make up the internet, AT&T's is a shared network, which means that the transmission links and other network resources used to provide the Service are shared among AT&T’s subscribers.

6.2.2.2 Connection:

Because service performance varies on an end-to-end basis, the service capability speeds of AT&T are limited to, and measured between, the equipment utilized to provision your Internet Service at the fixed address or location you identified when ordering the Internet Service and a point on the AT&T network, sometimes referred to as the “Connection” or your “internet connection.” The Connection constitutes only one segment of the end-to-end transmission path connecting the end user to internet web sites or content providers.

6.2.2.3 Expected Speeds:

Because there are many factors which may impact the speed experienced by any particular internet user at any particular time (as described in more detail below), the “Expected Speed” represents an anticipated, theoretical speed of the Connection, based on network design and engineering, measured over time. At any moment in time, a particular observed speed will vary from the Expected Speed. However, AT&T manages its wired internet network toward an overall median speed consistent with the Expected Speed.

I live in Indianapolis and have been having similar issues with peak time download speed reaching 30Mbps and I pay for the 1GB fiber. My last ATT tech replaced my ATT modem, and moved my circuit. Still speed from ATT modem diagnostic and smarthome manager report the same slow speeds and I can tell via streaming and just opening websites.

I really dislike Comcast, but at least I did not have as many download speed issues during peak times as I am having with ATT at this time.

Has anyone been able to get this issue resolved in your area with ATT?

Collect speed test results at various points throughout the day and record that data (screen snip is a great tool for this). I also reached out to several neighbors in my area who were on AT&T fiber to do the same. I then reported the issue to the FCC here: https://consumercomplaints.fcc.gov/hc/en-us/requests/new?ticket_form_id=38824

Within a day, I had the office of the president of AT&T contact me to help resolve the issue. I ended up running a few trace routes which showed that AT&T was having peering issues with their equipment at a node in Chicago. The issue was ultimately resolved, but it took a bit of time.