I just find mailinglist bad for keeping track of things, and even worse
for being a "knowledgeable" anyways (the web-archive navigation is
simply crap...) information getting out of data and obviously there is a
some noise (e.g. I asked recently for building draft PRs but only the
eclipse-infra was slow).
Let's not build false expectation: forums are as bad as mailing-list to store knowledge and to navigate. Both are unusable for knowledge archive without a serious search engine processing their content; and the list of discussions by forum is as bad as the discussion tree view of mailing-lists.
So for me keeping everything for ever is absolutely no requirement at
all... in fact there are not any 'hard' requirements at all,
It is not for you, but it is for many other people in the community.
It should even be possible to post every discussion/comment in
gh-discussions to the mailinglist as well if that sounds reasonable...
Having multiple channels for the same goals is harmful for OSS communities (we actually discussed that on the Eclipse Architecture Council as well as some projects are starting to suffer from being everywhere at the same time). So it is a requirement to focus on *replacement* instead of addition.
but starting to discuss about SLA, backup and recovery will simply make
using it most likeley never happen.
Sorry, but Tycho, like other Eclipse projects, is not a sandbox. It's a project that has a legacy and a desired long future, committed to a solid governance -such governance being funded by Eclipse Foundation membership-, some OSS SLAs and so on. You many not like to see your idea slown down by such requirements, but those are such requirements that make that Eclipse projects have a good longevity despite all the mess that can sometimes happen in community projects.
If it appears that GitHub discussions are likely to vanish out of the blue without reason or fail-back while -like mailing-lists- they become a source of documentation for some cases; then it means that despite all the hype, GitHub discussions are actually inferior to mailing-lists and probably note worth chasing.
That said, I'm not as pessimistic as you when it come to EMO SLA regarding GitHub services. You probably don't know that, but Eclipse Foundation does backup a lot of data from GitHub; I wouldn't be surprised if they were capable of doing backups of discussions already. This extra layer of professionalism is one reason that makes Eclipse Foundation projects more sustainable than the average GitHub repo.