Items in existing loot tables have become invalid causing backward compat issues.

Partner Rank: 3

Verification build: 1.16.100.57

Summary: Items in existing loot tables have become invalid causing backward compat issues.

Impact: this can cause issues for players especially if the intended loot table drop is important to progress and can break progression.

Repro:

1. Open Stranded in Space from the Marketplace
2. summon a pillager
3. kill it

Observed result: content log appears about invalid items even though those items should exist. this breaks backward compatibility with these maps, some of which cannot be upgraded for various reasons.
[Item][error]-minecraft:prismarineshard is not a valid item name.

Expected result: No content error log, item should drop, even if using an older syntax.

Screenshots attached: no