What happened to Sitecore Marketplace contribution?
What will be the alternative for that? GitHub?
How long the Sitecore Marketplace website will be available with all the contributed modules?
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The Sitecore Marketplace contributions needed to be retired for technical reasons and has not been replaced by Sitecore with an alternative at the time of this answer. The Marketplace site is still online to allow people to find previously contributed modules. We do want to solve for this problem going forward, possibly as part of the new Developer Portal (https://developers.sitecore.com) project.
We have been recommending GitHub as a great place to host your modules, and many other community members are already doing this. A lot of community members will have an associated blog article posted somewhere to explain/advertise the module. Make sure your README in your GitHub repo has the information needed for search engines to surface your module to people looking for it!
Sitecore hasn't made a decision yet to retire this site. At this moment, our team is hoping to keep this running as is until an alternative is available.