Hamster code has been moved to https://github.com/projecthamster
The app is under “hamster” (the name might change but i think it’s ok). And you can also see shell-extension there. If you happen to be hosting any of the hamster extensions for other targets, you are more than welcome to transfer the ownership and i’ll get you on the team as well 🙂
Right now it’s just the code, but i plan to move over also bugs and start releasing versions there.
I would like to thank the GNOME foundation for including hamster in gnome 5 years ago. It gave the project unprecedented exposure and the army of translators made sure it is available in all the major languages. A huge thanks for that!
As for the reasoning:
The gnome infrastructure is functioning perfectly fine. Just it doesn’t have the horsepower that is driving github and can’t possibly compete with all the development there. Apart from that, the hope that hamster will become core part of the desktop didn’t come true.
Also i think hamster will benefit from a more relaxed release pace and not having to comply to any of the freezes or dealing with any of the bureaucracy that simply doesn’t apply for a project code of which at the moment of writing 98% consists of work done by 2 contributors.
Project Hamster 
Why do you think you need the “horsepower” that github can provide?
You say that the codebase doesn’t move that fast and that it is primarily done by two contributors.
If there are specific things that have that gnome isn’t handling properly that might be something the infrastructure team would be interested in.
As liam said. And please don’t forget that you’re giving translators a hard(er) time with your move to GitHub.
currently all gnome infrastructure combined can’t get functionality parity with github.
the two big ones are pull requests and comments on commit lines. but there is much more to that. for example the security model of wiki and that you can switch the formatter backends to whatever you want. small things that make big difference when it comes to daily usage.
the other bit is that hamster was not embraced by gnome as hoped and so apart from translations, which are important enough, we don’t have anything to gain but have to comply to release schedules, contributing to it is much more tedious (compared to pull requests)
translations are an open question. if hamster isn’t gnome, should it be kept within gnome for translators? or perhaps there are other ways? i’ll have to look into that, if you have a working plan, i’d love to hear it
I’ve found Transifex pretty good for the translation side of things.