I know there's not much life left in this thread as the bigger secrets of the update have been roughly filled in on the wikis. I'd like to reinvigorate it a bit to take a deeper dive for map creators and automation players.
They have nerfed and will continue to nerf shimmer. 1.4.4.4 removed the shimmer invulnerability from boss damage, and they implied they'll do something for invasions next... Not sure why they didn't just do the same for certain invasion enemies already. The money exploit of buying and selling the materials from timers and copper tools was also "fixed" by blacklisting them from shimmer uncrafting... I hate this method, better to change what's given back or adjust prices. I based my whole shimmer skyblock map on the understanding that people could break down their axe and shortsword to have 33 dirt blocks and 6 platforms available..... anyway, I gave up sleep and made a workaround.
I've noticed some crazy unique behavior to the secret seeds. Firstly, The Zenith seed seems to do almost nothing code-wise by itself, It changes the world icon, and enables the flags for all other secret seeds at once. In TEdit you can edit which ones are toggled on or off. Most of the Zenith specific behavior comes just from having For The Worthy and Remix toggled on together, with or without Zenith toggled on. Examples: I mentioned the lihzahrd brick no longer counts as jungle in Remix. If FTW is also on, they aren't needed for spawning lihzahrds and flying snakes. They spawn on any blocks if the player is behind natural lihzahrd wall (good news for skyblock). Normally they'd still only spawn on their bricks. Normally lava slimes drop lava in Expert+. Remix stops that. Add FTW and lava bats drop it instead, but on all difficulties.
Remix does a whole lot by itself, and I still have questions. I'm still wondering about what changes at the surface other than worldgen. In a normal world, the surface updates growth and biome spread 6x faster than underground/cavern/hell. Does that apply to the remix seed or is it the upper cavern that gets boosted speed to go with it's weather effects? EDIT: Confirmed while streaming my map just a bit ago that indeed, the true surface does still grow plants and do biome spread 6x faster.... I didn't test if there was any exception to that at where spawn would be in the underworld, but I doubt it.