How to reproduce:
1. Open the attached project's Scene labeled "SampleScene"
2. Go to the Publishing Settings (Edit->Project Settings->Player->Publishing Settings)
3. Make sure that "Compression Format" is selected and "Decompression Fallback" is enabled
4. Make a WebGL build
5. Open the WebGL Player in a browser
6. Press the "Launch Unity" button (*WARNING* Don't wait for the Player to fully load as it freezes up the system)
7. Press the "Quit" button
8. Take a Memory Snapshot (press the shortcut F12 and go to the Memory tab)
9. Inspect the used Memory size
10. Repeat from the 6th step a few times (2-3)
Expected result: Memory is released after quitting from the WebGL Player
Actual result: Memory is not released after quitting from the WebGL Player
Reproducible with: 2020.2.5f1, 2021.1.0b7, 2021.2.0a6
Couldn't test with: 2018.4.32f1, 2019.4.20f1 ("Decompress Fallback" option is not available)
Note: Not reproducible with no compression and "Decompress Fallback" disabled