Depends where you are. Where AT is rolling out (WCML and ECML) it will be because they've not got round to removing them yet; GE and LTS it will be because the RC has been stolen between them so the boosters have been disconnected.
There was a plan to AT GE as well, I can't remember whether this is still happening or not.
The AT-in of the GE is still happening between Pudding Mill Lane & Shenfield, as part of Crossrail, which is also covering the GWML between Royal Oak and Maidenhead. When both sides will become operational is anyone's guess!
The ECML AT feeding is only between Welwyn & Hitchin at present; everything between Wood Green & Bawtry (nr Doncaster) has been converted to Boosterless Classic feeding as part of Phase 1 of the ECML PSU. Phase 2 will cover the rest of the route to Edinburgh, but the final scope has yet to be agreed.
As discussed in some of the dedicated PSU (Power Supply Upgrade) threads, the WCML AT-ing programme was scaled back due to 'value engineering' (coincidentally, the same thing happened to the ECML, albeit in the design phase). Currently, North Wembley (the original southern limit) to Bourne End has no functional return wire at all (as all masts are bonded, and a Return Screening Conductor is in the signalling troughs) as Bushey ATFS was mothballed. Some lengths of ATF wire are up in places, however.
Bourne End - Whitmore (and Stone) is complete, Whitmore - Weaver Jn still uses RC/BT, Weaver Jn - Euxton Jn is complete, Euxton Jn - Carnforth still uses RC/BT (but has ATF supports up in sporadic locations), and Carnforth - Great Strickland is complete.
