Breaking my posting-block to at least add some insight to alternatives for fallout refugees. I hope this can be read before (if) current existential threats come to pass. Trying to be as neutral as possible (besides the furbooru note and a shot or two over e621’s bow below) in this list. These are all that my sleep-deprived mind can think of, including a few reaches.
ponybooru.org - Not to be confused with brony.booru.org, also titled Ponybooru. Apparently opened 6 days ago (as of this post, going off of rules edit history). So far appears to be best alternative, appearing to be a direct clone of pre-fallout Derpi running on Philomena. Still in “beta”, so who knows what may change over time, including rules (which is currently stated in the rules as well), but has policy framework in place. Extremely asinine DNP/takedown procedure.
ponerpics.org - Apparently opened 5 days ago (as of this post, based on About page revision history). Another Philomena-powered Derpi clone. Rules are extremely sparse, but so far lean toward freedom and archival use. Appears to be best fallback after ponybooru. Only lower in this list because policy framework has not yet been fleshed out.
rainbooru.org - Around since the start of 2018 and still looks like ass and its server hamsters are underfed, but it’s serviceable if clunky. No rules to speak of, short of “no illegal stuff”, which could become an issue if rules are suddenly added after an explosion of activity (/b/-day flashbacks keep running in the back of my mind). Has a canary, but no signature system to verify authenticity, rendering it effectively moot. Has a derpibooru-specific importer.
brony.booru.org - Not to be confused with ponybooru.org. Opened in 2012 and effectively abandoned. Powered by Gelbooru and hosted via booru.org. Booru.org has an over-arching ToS that has not been updated since 2010 with anti-loli and anti-watermark and vague “anti-obscene” clauses, 404’ing external links on “network” pages, injects their project patreon as an ad on all network boorus, and generally feels like it’s crumbling even while raiding and shutting down its own hosted boorus at random. Listed here only to note that it should be avoided.
furbooru.org - Only listed here to mention as an avoid. There are concerns about current/past/hidden staff; I am not bringing this up to start arguments, only to be thorough. Avoid.
e621.net - Staff have always been ass, tagging becomes more complex with each millisecond, has an anti-watermark clause in rules, is certainly anti-fun (no memes, mouthpieces, etc.) and is obviously all-inclusive furry and an extremely hugbox community. But hey, it’s there, and has been for over a decade.