Does it fill in the new unregistered shallow water aka marshland or mud terrain? Normal landfill does not fill this is. If you could add it in that would be Terrific!

I'm not sure. Do you know the internal tile names of them?

This is the list of affected tiles from the mod's control.lua:

local candidates = {
"water",
"deepwater",
"water-green",
"deepwater-green",
"water-mud",
"water-shallow",
"wetland-light-green-slime",
"wetland-green-slime",
"wetland-light-dead-skin",
"wetland-dead-skin",
"wetland-pink-tentacle",
"wetland-red-tentacle",
"wetland-yumako",
"wetland-jellynut",
"wetland-blue-slime",
"gleba-deep-lake"
};

I got that from the list of tiles that landfill claims to work on in the data.raw. So if you're asking about tiles that landfill cannot fill, then no, it does not fill them. (Note that this mod doesn't fill anything by itself, it merely orders bots to do so.)

A natural extension would be to also order the use of foundation on tiles that only that works on. Is that what you're asking for?

I found this in another post about this issue.
"
table.insert(data.raw.item.landfill.place_as_tile.tile_condition, "water-shallow")
and
2.0 without mods you can see the Factoriopedia entries. Landfill can only go into Water, Deep Water, Green Water, and Deep Green Water. But not into Shallow Water or Marsh.

The link
https://forums.factorio.com/116474

"water-shallow" is indeed in the list of tiles to be handled, so that should work fine. I don't know what "marsh" refers to though (unless that's just an alternate name for "water-shallow").

https://mods.factorio.com/mod/ShallowFillFix has saved me!

Glad to hear it's been resolved! I was thinking that a fix along the lines of what ShallowFillFix does might be needed, but I didn't know how to reproduce the problem.