Some of the older legacy versions of Atlassian SDK are no longer hosted in Atlassian’s maven repository.
Where it’s important to keep them, I’ll subdirectories from https://maven.atlassian.com/content/repositories/atlassian-public/com/atlassian/amps/atlassian-plugin-sdk/ into this directory and serve them via RawGit.
The last time I looked, these where the versions which were available:
Index of /repositories/atlassian-public/com/atlassian/amps/atlassian-plugin-sdk