The only iPads with severe battery life issues are those that have been through a quintillion iOS updates: A8, A9 and A10 models for now. So, the Air 2, 1st and 2nd-gen iPad Pros, and the 5th and 6th-gen iPads. Those are the devices that I see mentioned the most with battery life issues. The combination of several years of use coupled with 7 major updates is definitely not good for long-term (as far as iPads go, 7 years is long-term) battery life.
I agree that cycles are not that relevant. In fact, I’ve seen 1st and 2nd-gen iPad Pros which have been very lightly used and battery life is still abhorrent. I’ve seen some with maybe 400 cycles and battery life is very poor. 400 cycles isn’t anywhere nearly enough for them to account as a remotely impacting factor in battery life.
Earlier iPads have far larger batteries and 32-bit iOS versions are far better, even fully updated. I think as far as long-term iPad battery life goes, up until the iPad Air 1 (this one a lot less than the iPad 4 and below, but still), they retain good battery life. It plummets on A8 through A10 iPads, due to the combination of iOS updates - like I said - and smaller batteries. Full-screen iPads? Remains to be seen. It will be interesting. Honestly, I haven’t seen a report yet of a 3rd-gen iPad Pro with abhorrent battery life. The other interesting factor is base iPads from the 7th-gen: larger batteries, more efficient processors. Will they suffer too eventually? Time will tell.