I tried to add a redirect for the first time and it didn't apply at all. Here's what I added:
^https://www.reddit.com|https://old.reddit.com
For some reason, disabling personalised results and enabling it again makes the redirect work. This behaviour happens with every new or modified redirect.
Redirects should apply immediately without requiring to toggle personalised results because that is not really self-evident. Also, disabling personalised results isn't even available when results don't have a site blocked.