I think it's to allow time for the losing CP, who gets notified when the new provider orders the transfer, to contact you and get a response confirming you desire the move. Several phone rental movers have reported such communications from the losing CP.

It's an anti-slam thing, as if you reply that you don't want to move, they can block the transfer.

Was told that over 30m can create problems, I do now if you artificially attenuate a line, for instance add 40db load on the line, the kit I use fits between the DSL out port of the filter and the tester, then any use of the PSTN side of the filter will cause errors and loss of sync. So I suppose some boffin has found this to be the limit at which it can start to degrade things.

Though as Bob says, the original data extension kits were in pre made 5, 10 and 30m lengths, it now just comes on a reel, and we fit the RJ11's ourselves.

Ethernet can run over 2 pairs

I'm sure it can, but Openreach only use 1 of the pairs.

Is it still supplied with an RJ11/12 connector at one end and a socket to connect at the other?

No, you get a bag of RJ11's and a crimping tool. The cable now terminates in faceplate filter, and you fit the plug to go straight in to the VDSL modem.

I use the cable with RJ11s at both ends for the short filter to modem link - have done for a while and when on ADSL found it gave a better sync over and above the flat cable supplied resulting in an extra 500kbps.