My understanding is that frontier serves mostly rural customers using the POTS infrastructure. They were advertising internet speeds that they were never capable of delivering.
This is indeed correct. While I understand the predicament of being far from the switch reduces speeds, it’s a bit absurd when you’re paying for 12, but can only get 3. (This is not an exaggeration.)
You’re are lucky to even get 3 if you pay for 12. In my (small) hometown most people have frontier so I definitely gathered a little more an anecdotal evidence when working on people’s networks/PCs and it was usually in the kilobytes 1-3 mbps on a good day (usually the lower end.)
At least in my area, most DSL providers derate their offerings based on distance from the DSLAM, so they're transparent about this... e.g. punch addresses in a small town into the CenturyLink website and you will see the offering decay from 15mbps to 3mbps as you get further from the phone exchange. I'm surprised that Frontier wasn't doing the same, or perhaps they were but being far too optimistic in their estimate.
Frontier also serves a bunch of reasonably large cities (for example territory they purchased from AT&T later in life). Where I live, I can get Frontier's "up to 24Mbit/s" service (probably closer to 10 if I'm lucky) or the competitor (cable company) which offers both traditional cable AND a new FiOS service with speeds up to 1 Gigabit. I haven't upgraded yet, and am still on 200 down/35 up but pay a fraction of what Frontier would charge (and I usually get closer to 215 down and 36 up). It's no wonder Frontier declared bankruptcy.