Don't do that. The results are worthless. Maybe >100.00 working pihole installations exist, only 16458 registered users in that forum, even less are active. Only a few will take part in the poll. The sample will never be representative.
Personal guess: Most users don't configure the MAXDBDAYS setting and leave it on default as they a) don't know b) don't care. If they know and care about, they've probably also read the release notes. Reducing the default in a major version leap should therefore be relatively safe.
I would go for 90 -180 days for default. I think that's enough to see past performance. 30 days is too short, in my opinion. In the various help topics we can't get adequate info in 30 days.
I have all my installs at 180, since I found that was as far back as I ever went. Typically I go three months back for investigations.
I don't think 180 will be much of an improvement compared to 365. You gain basically nothing . Half the disk size is 3dB, far away from an order of magnitude. My vote is for one month maybe slightly more as suggested by @jpgpi250 as default. However, it makes me sick in the pit of my stomach to read long-term and then the data is actually only one month. This doesn't seem to fit together. One year is clearly more long-termy.
Does this really need to get changed at all? Just trying to find out how many users actually complained about this. And if they did, you can easily suggest using the option. More users that you think may use this from time to time. And maybe more users than you think are happy to be able to go back to February and check what was happening then. When we delete after 180 days, most of the February would already be gone.
So, again: Does this really need to get changed at all?