A search for Player Health Indicator Mod 1.21.8 should begin with one question: does the project actually publish a build labeled for 1.21.8? For the original PlayerHealthIndicators project covered here, the latest published release shown in its history is 1.1.1 for Minecraft 1.21.
Check the release label before the filename
A renamed archive can say almost anything. Reliable compatibility comes from project metadata and an official release target. If the release page does not name 1.21.8, do not let a third-party filename override that absence.
Match the whole runtime, not just Minecraft
For the current 1.21 project baseline, the metadata names Fabric Loader 0.15.11 or newer, Java 21 or newer, and Fabric API. A valid future 1.21.8 build would still need a compatible loader and dependency set.
Separate startup failures from rendering failures
If the game will not load, investigate loader, version, Java, dependency, and duplicate-mod problems. If the game loads but hearts are missing, investigate the mod toggle, rendering conflicts, and player visibility conditions.
Keep exact-version support explicit
Until an official build specifically targets 1.21.8, present the version as unconfirmed for this project. That is more accurate than assuming that all 1.21.x releases are interchangeable.
Next steps
Use the homepage for the complete feature, release, FAQ, and download overview. For a clean installation workflow, read the installation and troubleshooting guide.