This mod adds a command that allows the player/developer to print a list of all items/blocks/advancements that fit certain args:

/listplease <items|blocks|advancements|help> [filters...] [chat|file]

Subcommands

items → search item registry
blocks → search block registry
advancements → search loaded advancements (excluding vanilla minecraft ones)
help → prints usage

Filters (all optional)

modid:<id> → restrict results to one or more namespaces
- multiple allowed (modid:minecraft modid:create)
#tag → restrict results to entries with a given tag (items and blocks only)
- multiple allowed; intersection logic is applied (must match all tags)
contains:<word> → restrict results to registry paths containing the given word
- multiple allowed; all must match (AND logic)

So, putting all that together, some examples:

/listplease items modid:minecraft contains:sword chat → all vanilla items with "sword" in ID, printed in chat
/listplease items modid:minecraft modid:create contains:door file → all doors from Minecraft and Create, written to file
/listplease blocks #minecraft:logs #minecraft:planks file → all blocks that are both logs and planks, written to file
/listplease advancements modid:ars_nouveau contains:glyph chat → Ars Nouveau advancements containing "glyph", printed in chat
/listplease items file → all items from all mods, written to file

Modpack developers may find the "print to a .txt list" feature useful for making quests. If you're just a regular player, JEI/NEI/EMI is almost certainly better.

You can use this in your modpack, with or without credit, but since it's mainly for developers, I'm not sure why you'd leave it in the final product?

List Please!