Actively collaborating (submitting Pull Requests to the GitHub mirror, sending patches via Gerrit
Contributions to Haiku code are NOT via Pull Requests to the Github mirror. Github is used as a mirror only. Haiku project as its own git repository, which IIRC is already hosted outside US, in the The Netherlands. Same for its Gerrit and Trac servers.
One issue, though, is to use Haiku Gerrit you need to authenticate via a github account.
Another issue is the hosting company, DigitalOcean LLC, is an US company, so their datacenters in Netherlands are not immune to any US extraterritorial laws and/or sanctions.
Anyway, notice that there are other mirrors of Haiku code beside the one on github. One is on codeberg.org, hosted in Germany, for instance.
For communication (mailing list, IRC, mail, this Discuss forum itself, Telegram channel, alt.os.haiku usenet, etc), some may be currently under US regulations, but never all could ever be, so one will always have a way to exchange.
Haiku, Inc. (a U.S.-registered nonprofit) and U.S.-based core developers are legally required to comply with these rules.
Haiku Inc. doesn’t own Haiku code or the project itself. It only owns the Haiku trademark and can and do fund some actions regarding Haiku project. Would it be facing US sanctions, it will impact maybe the trademark validity in US and its ability to collect funds and use them, and potentially US-based individuals registered to Haiku, Inc. could have some legal exposure.
But not the Haiku project itself.
Regarding any US-based contributors, well, like any US citizen that are under US laws anyway, by definition.
But Haiku is not just Haiku project. It’s also HaikuPorts and HaikuArchives, and here there is a bit more reliance on Github currently.
But, again, may the situation worsen enough to become a necessity, it’s open source world, people will switch to mirrors, turn them as active repositories and upload theirs git clones to alternative git hosts in such case.
It’s the nature (and power) of open source projects. It’s, by design, in the open, for anyone.