This list by @oliphant is all the instances that mastodon.social doesn't block, that 'we' (the instances who use Oli's blocklist, which in part pulls from .art's blocklist) do block in the unified_tier0_blocklist.csv file.
You can look at those entries and then do a search on https://mastodon.art/about to find the instances and see the reason we have them blocked (most of them are there).
(continued)
If you have friends on mastodon.social and mastodon.online who belong to marginalised groups, who need online safety and protection from targeted harassment, perhaps they should be made aware that the instance they're on is not the safest.
The moderation on those two instances has improved in the past few months in terms of dealing with individual account reports, but they're still clearly sorely lacking in the fediblock department.
Mastodonâ˘ART đ¨ Curator