I'm using the latest version of your mod (1.15.2-3.1.1) and wood plank recipies for modded wood won't generate. The vanilla minecraft recipe (1 log -> 4 planks) works but when I try to use the flint axe or saw recipe there's no output. I have JEI and the modded recipes don't show up there either. The logs have all the minecraft:logs tag. Is there something I need to do on my side to make it work?

Mods I use that add new trees/logs:
Atmospheric
Swamp Expansion
Upgrade Aquatic
Endergetic Expanion
Autoimmunity
Sakura Rosea
Bloomful
World of Wonder
Dynamic World
Misc Additions

None of the flint axe/saw recipes work with the wood added by these mods.

Short answer, yes, this is intentional. Recipes are data packs now, and short of explicitly adding recipes for these specific mods, they will not be automatic. Recipes for logs -> planks can't just use tags as otherwise you'll be making any birch planks from spruce logs and other nonsense.

Recipes for sticks from logs or sticks from planks are automatic and simply use the minecraft:logs tag.

What do I have to do to add the recipes?

Make a Data Pack which adds the required recipes, and removes / replaces the existing recipes from other mods.

For example, with acacia planks:
Removing the vanilla recipe
Log -> Planks with flint axe
Log -> Planks with saw

Do I place the data pack in my mods folder when I am done making it?

No. To quote the documentation article I linked, (please do read that it will answer all your questions).

Data packs can be placed in the .minecraft/saves/(world)/datapacks folder of a world. Each data pack is either a sub-folder or a .zip file within the datapacks folder. After it is in the folder, a data pack is enabled for that world when the world is reloaded or loaded.

Thanks 😊