Slits can be sewn after the tapestry is finished and still under tension on the loom or it can be removed from the loom and then sewn.
The latter, used at the Scheuer Studio in NYC in the 80’s and 90’s employed a strong thread that joined the two warp threads on either side of the slit with a figure eight type interlacing (figure A) hidden under the warp. It was executed with a curved upholstery needle and sewn from the front.