It's time to make a decision for Spurs:
If Biyombo isn't waived by January 7th, his contract will become guaranteed for the rest of the season. Same is true for Waters.
Since Biyombo is currently Spurs' fifth string center behind Wembanyama, Kornet, Olynyk and Sochan, Spurs might waive him and not replace him with a center.
A possible scenario is Spurs waiving Biyombo and letting the 15th roster spot open until the February trade deadline. Spurs are right now $5.7M below the tax and if they waive Biyombo without replacing him, they will be $6.9M below the tax. Spurs might want to keep that flexibility for a potential trade at the deadline involving Sochan/Olynyk/McLaughlin/Waters that will raise their team salary.