For me, it depends upon whether I'm using the bestiaries for a D&D 5e campaign set in Midgard, or just using them to enhance a campaign set in another world. About a 1/3rd of the 300+ monsters in the original Tome of Beasts had terrific sidebars with flavor text for monsters with charateristics that were pertinent to the World of Midgard. WIth the big success of the ToB1 kickstarter and follow up sales, it became apparent that many, if not most, DMs were using it to enhance campaigns set in worlds other than Midgard. So beginning with the Creature Codex, Kobold Press more or less stopped including the Midgard specific side bars.
I copied & pasted that flavor text into the monsters/NPCs in the Midgard campaigns I ran with a VTT, and would read it out to my players where appropriate during encounters. They really enjoyed that and TBH I really dislike that KP stopped including any Midgard flavor text in their later bestiaries, but I'm no doubt among the minority of DMs. ToB1 is my go to book when I want to homebrew encounters or adventures for a Midgard set campaign. That said, IMO all four have quality threats that will enhance any 5e campaign.
I'm not sure what you're referring to as the non-5e content in the OP, but KP's earlier threat books that were published for PF1e, D&D 4e, 13th Age, Fantasy Age (I'm sure I'm missing a few more), all had the title of the specific TTRPG in their name; i.e "Midgard Bestiary 13th Age Compatible Edition". Those contained fewer monsters/NPCs than the ToBs or CC (about 100 per book), but still contained many quality threats. Any of which with a bit of reskinning, could be placed into just about any other fantasy setting.
