The bug

When starting the game the resource packs information in the options.txt file is checked and three cases are detected (shown here using the log messages):

Removed resource pack {} from options because it doesn't seem to exist anymore

Removed resource pack {} from options because it is no longer compatible

Removed resource pack {} from incompatibility list because it's now compatible

However, despite all of them saying that the options were modified, this has actually no effect because the modified options file is not saved afterwards (unless the user changes a different setting).

How to reproduce

1. Select a resource pack ingame
2. Close Minecraft
3. Delete the resource pack
4. Start Minecraft and look at the log
   A warning is logged which says the resource pack was removed from the options
5. Close Minecraft
   The options file was not saved and the same warning will be displayed the next times as well

Code analysis

20w08a, Mojang names
Method: net.minecraft.client.Options.loadResourcePacks(PackRepository<UnopenedResourcePack>)