The thrall cap feature was introduced more than a year ago in the game and has been available for server admins and SP players since then. Enabling it on official servers is a setting change that would require nothing but a server restart. This is why it is not included in the patchnotes.
However, once we have concrete information on the exact date it is going to be enabled, we will release an announcement post with all the details in advance, including the final number per server type as well as any other useful information.
The initial plan is to have it enabled once, or shortly after, 2.4 is released. But we’re still discussing internally the fine details to every aspect mentioned above.