1. I think the opt-in should have been there at the start. Repo maintainers should have been informed, and given ample time to decide if they want to participate or not, so that participants are likely to have a wider range of repos to choose from. From the organiser's POV, it will help them to measure the impact of this event on open source events. However, this doesn't seem to be planned at the start (didn't happen in the past years either?) The explanation I can think of is that either they didn't have a nice talk with GitHub to get their support, or like many ppl have said, it's more of a publicity stunt.
2. It could have been done with better communication. If not for this HN thread, I wouldn't have known the change in rules. No email sent. Even the official website still has rule sections that say any public repo will work.
3. I'm discouraged to participate further. I do love the t-shirt. I'd be lying to say that I wasn't motivated by the shirt from the start. I also wanted to beat the crowd by making PRs early by sacrificing my time for other hobbies/work. However, I made PRs that are welcomed and the maintainers were happy to merge. But now I feel the shirt will only bring me the suspicion and disgrace of potentially being a spammer.
4. I don't have many choices of repo to contribute now. In the hacktoberfest topic, there's 0 repo in my preferred language. The first repo I see with the highest number of stars is some algo repo. The 2 repos I've contributed and likely to contribute again are not qualifies anymore.
There are still good things from my experience. I have made PRs before, but I'm not actively contributing. I'd rather spend my free time on other hobbies. But now, I do see that I can find joy in open source. I'm likely to contribute to repos that interest me again in the future.