Its been talked about quite a bit here but the backstory of how the 21 doesn’t go to Union anymore has to do with last summer. Traffic was insufferable on weekends to the point where getting into downtown took almost 2 hours to get into and there was also construction near the Union bus terminal at that time which made matters worse. 21 buses were then all rerouted to detour to Port Credit, along with 31 buses as well (this was way before the Kitchener weekend train service started), which made matters even worse because of the Hurontario LRT construction and QEW traffic.
So GO earlier this year gave their solution to this problem by these Milton bus reroutings and finally introducing the Kitchener corridor weekend trains. These buses going to the LSW stations were nice at first but now its only useful going anywhere else in the GTA but DT Toronto which is obviously where most people are going and where most of the complaints come from when discussing the Milton corridor bus reroutings. The connections to the 407 corridor buses and the LSW trains actually do work out well though.