They're not blocking uBO, they're removing the features in the browser that allowed uBO to work by releasing new plugin APIs, "Manifest v3". They're eliminating the key APIs needed for uBO to identify things that it shouldn't load, and then not load them. Google claims this was for "performance" or "security" reasons. Of course, the only major 'performance' or 'security' affected is the ability to identify, intercept, and stop harmful or ad related downloads before they start.