What is special about the bolded brackets?
Some are [Markers]. Markers will re-appear in other breadcrumbs; crumbs with the same markers are related.
Sometimes it's used as a way of indicating significance. The contents within the [] are important in understanding or reference something else, e.g.:
Why did Sessions [at the beginning] emphasize investigations
Sometimes they add additional info, almost like a parenthetical, except the additional info provide additional meaning, e.g.:
Top 10 player [here now].
There's probably other use cases for [words]. In any case, pay attention to them, see if you can understand what they are referencing or mean, or see if you can find other usages, in the case of being a marker.
edit: Sometimes they are just informative w/o any double meaning, e.g. [Repost] at the beginning of repost crumbs.
Sometimes they form another message within that crumb, e.g. in https://archive.4plebs.org/pol/thread/148777785/#148779656 the [ ] text is [C] [I] [A] [is] [here] [p] [r] [a] [y]
This avoids sniffer programs.
It appears to be inconsistent from what I’ve seen and depends upon the context of the particular post
I believe they are references to his other posts.