All currently known Wii U exploits can, unlike e.g. the Nintendo Switch RCM exploit, be patched by a system update. Although the Wii U is no longer officially supported, Nintendo may still release updates for it. Namely, the updates 5.5.3 up to 5.5.6 were all released after the Wii U was discontinued, so blocking updates is still a recommended action.
While Tiramisu's PayloadLoader already has built-in update blocking functionality, it is recommended to delete the update folder to effectively block system updates. If you get a red warning screen while booting into Tiramisu, the update folder still exists and it is recommended to delete it using the guide below.