Minecraft wiki entry for Wither_Skeleton, associated text: 0 to 2 bones. The maximum drop is increased by 1 per level of Looting, for a maximum of 0 to 5 bones with Looting III.
There is a 1⁄3 chance to drop a single coal. Each level of Looting increases the maximum drop by 1, so that Looting I drops 0-2 coal with an average of 0.75 coal per kill, Looting II drops 0-3 coal with an average of 1.2 coal per kill and Looting III drops 0-4 coal with an average of 1.67 coal per kill.
2.5% chance to drop a wither skeleton skull when killed by a player or tamed wolf. Looting increases the chance by 1% per level‌[Java Edition only] or 2% per level‌[Bedrock Edition only], for a maximum of 5.5%‌[Java Edition only] or 8.5%‌[Bedrock Edition only] chance with Looting III. Their skull is always dropped if killed by a charged creeper.
8.5% chance of dropping their unenchanted stone sword which has a random durability. This chance increases by 1% per level of Looting, for a maximum of 11.5% chance with Looting III.
5 experience orbs when killed by a player or tamed wolf. If it has armor, an additional 1 to 3 is dropped per piece.
If a wither skeleton wearing a carved pumpkin or jack o'lantern is killed using a tool enchanted with Looting, there is a chance equivalent to the level of Looting used to drop the carved pumpkin or jack o'lantern, a maximum of a 3% chance of a drop.