I'm playing Scrappy Chemistry with Angel's Petrochem and Space Age (and a bunch of other mods)
I'm using the remix recipes.

The recipe Sour gas sweetening can't be used in the chemical plant.
In Factoripedia, I found the recipe, but it has no technology to unlock it.
I'm assuming, that the prerequisite "sulfur-processing" got overwritten by Angels Petrochem:

OV.global_replace_technology("sulfur-processing", "angels-sulfur-processing-1")

Thus leaving Sour Gas Sweetening without a technology to research it.

I also encountered this problem with the Impure fuel and Coal digestion recipe, so I guess all recipes which require sulfur processing are not available.

Is that something that has to be fixed in Scrappy Chemistry, or should Angels Petrochem ensure that all recipes get moved accordingly?

I feel that if a mod is going to totally replace a base technology then it should probably make sure that recipes added to that base technology are moved somewhere accessible. In practice, it's a bit more blunt, we don't use Angel's and don't have any plans to add support for it, so this won't be fixed on our end.

Thanks for the explanation.
I've written a bug report to Angels: https://forums.factorio.com/viewtopic.php?p=694251#p694251
(currently still invisible, until a moderator approved the post)