Not for FTTC - It's now in their T&C's which you are supposed to read and agree to before you sign up.
That's not to say that some people don't break the rules
I'm not sure as I don't use Sky... but don't you either need a MER modem/router or run in bridge mode?
yes its allowed kitz. --> edit-->
I agree with kitz now, reread it and yeah thats clear sky specifically consider these devices not allowed.They dont support it, but thats very different to not allowing it. I know people can confuse the two things but they not the same.
Actually locking down a modem, look at Virgin Media, they have customers stuck on PUMA devices that cause constant latency problems, but they are locked to those devices via MAC enforcement and would be breaching t&c if they swapped the devices.
Also the sky hub even allows you to use an openreach modem on their setup wizard (any compatible modem, but the image shows a hg612). It has a WAN input connection for this purpose.
MER has absolutely no affect on the modem. For the router, it has an affect and yes is a limited set of routers/firewalls that can technically support it, such as PFSense and OPNSense.
To the OP check my sig to see hardware I am using on sky right now.