Yes, they're still open. Sleaford East at the Skegness end and Sleaford West at the Grantham end. In the area all boxes from Allington (new) box to Skegness are open (only recent closure was Barkston East Junction about 12 years ago with Thorpe Culvert and Allington being rebuilt - last closures before that were 80s/90s). Boxes are Allington, Ancaster, Rauceby, Sleaford West, Sleaford East, Heckington, Hubberts Bridge, West St Junction (Boston), Sibsey, Bellwater Junction, Thorpe Culvert, Wainfleet and Skegness.
On the Joint line between Peterborough and Lincoln the only surviving boxes are Littleworth and Lincoln High Street (both out of use) following the demolition recently of the former Sleaford North box. Nowadays the line is controlled by Lincoln SCC except where it dives through Sleaford, fringing off the top of my head to Peterborough PSB at the south end and Gainsborough Trent Junctions at the north end. Stow Park and East Holmes boxes also survive as listed structures but out of use.