I admit i'm conflicted about this as well. I loathe that lack of attribution in Tumblr culture myself (one of the reasons why I ask in my profile not to repost my own stuff on Tumblr), and I'd find it less than ideal for recs here, but wouldn't completely rule those out either, if the artist original posting place really can't be found with reasonable effort.

What I mean is, if someone sees cool art without attribution reposted somewhere and wants to rec that here, before doing that the reccer should at least look closely for sigs and whether maybe a google search turns up the artist (for example if someone saw my art reposted somewhere without linking back to me, they could do a google search on my sig and find my page), or depending on how the reposting was done, whether the URL still may contain the original filename and search for that shows another place and such (maybe even asking on a finder comm if their fandom has such infrastructure), and not just going with the first Tumblr repost they saw merely for convenience, without even trying to find the artist.