Yes, it used to be the case that Sky had, in their terms and conditions, a point that said that 3rd party equipment was forbidden.
However, they didn't stop anyone who actually did. I very quickly dumped the Sagem router when they first gave me FTTC in 2012 and nobody has ever said anything about the non-Sky equipment.
Obviously it became interesting if I ever needed to call them, but luckily that has only had to happen on very few occasions. I had Sky Q in early 2016, and as an early adopter, I think the engineers were expecting to see the Sky Q router in place before they installed the TV due to the Wi-Fi mesh. As soon as the engineer left my house, their router was promptly removed and I put my own equipment back on.
I think in the last few months they have indeed relaxed that requirement, I believe there was some Ofcom ruling that has forced this, but don't quote me on that as I am not sure if I actually read that or if I'm making that up.
Having said that, I don't know if they publish anything to help you achieve that, my guess is not - if you wish to use your own equipment then go ahead, but I don't think Sky would be expected to support whatever esoteric routing device you have on your network.