I think it's *probably* fair to leave PLC off the threat list for B-Index , since, as you say, it can be backed up (though I think one would have to get control of the domain to actually use the backups in a disaster). I'll focus on the relays.
And yeah the bad space and others like it are why I want to figure out how to include blocklists in the calculation for the fediverse; the hard part of course will be trying to get data on how widely they are used. I've been watching the bad space from the beginning and have my own concerns too. I *think* the tier0 lists from seirdy https://seirdy.one/posts/2023/05/02/fediverse-blocklists/ and oliphant https://codeberg.org/oliphant/blocklists are more widely used, but frankly that's just a hunch, I want data. :) And whether you think seirdy's list is good or not (I happen to think it looks good), it's one person maintaining the tier0, so that significantly centralizes things for the B-Index for the fediverse.
BTW I just added git forges to the main stats page...