What I expected to happen was...:
When "Historical versions" and "Enable snapshots" sliders are turned on, its supposed to display a list of Minecraft versions created so far in the launcher.
What actually happened was...:
Some Minecraft versions are missing in the launcher.
Steps to Reproduce:
1) Open the Minecraft launcher.
2) Log in to Mojang if you haven't.
3) Go to the "Launch Options" tab.
4) Turn on the "Historical versions" and "Enable snapshots" sliders.
5) Click on the "Add new" button.
6) Click on the "Version" drop-down list.
7) Scroll down all versions.
8) Notice that version 16w20a is missing. I tried to use 16w50a, but it didn't work.
I believe older snapshots are removed from the version manifest deliberately. I'm not 100% sure why, but I think @unknown said so somewhere.
You can manually add them via the s3 URL: create a folder in
versionsnamed16w20a, then save https://s3.amazonaws.com/Minecraft.Download/versions/16w20a/16w20a.json in that folder, and then launch that. Note that the launcher will purge unused snapshot versions if you don't have a profile for them (again, I believe that is deliberate).