Which means that they have indeed cheapened out with a recent refresh - which is a shame. I have the cable I describe on the headphones I am quite literally wearing on my head at this very moment (HD 600) and identical ones were present in 59x and 650 from the era. Glad to know, TIL.
Edit: the 3.5mm male to headphone is Sennheiser 81435 [0] the 6.3mm male to 3.5mm female that fits over it is Sennheiser 562507 [1].
I bought the HD600 last year and it came with the 6.3mm to 3.5mm removable adaptor. This was in Europe maybe the cabling is different, or there are multiple packagings for different use cases (likely). The cables are removable so its easy for them to have different SKUs.
HD 559: https://assets.sennheiser.com/img/18214/product_detail_x2_de...
HD 650: https://images-na.ssl-images-amazon.com/images/I/61vm4c%2Bhb...
HD 660 S: https://assets.sennheiser.com/img/25816/x1_desktop_sennheise...
HD 800 S: https://cdn.mos.cms.futurecdn.net/cVSs2jBo28NYKPkdGtZhnR.jpg...
I'm not seeing a lot to back up your claims. Especially since a lot of those headphones come with a 6.3 mm to 3.5 mm adapters.