Linux

Yes, absolutely. And they can drag Canonical into it as well if they wish though it's harder. Being UK based doesn't protect them from the long arm of US law including arresting any US personnel, freezing and seizing their funds, putting out arrest warrants for and harassing those in the UK with the fear of arrest and rendition to the US if they go to a third country (for a conference, vacation, etc, most would buckle rather than live under that). Additionally the US could sanction them for non-cooperation by making it illegal for US companies to sell them products and services, for US citizens to work for or aid them, etc.

They can go after community led projects too, just send the feds over to the houses of some senior US developers and threaten and intimidate them, intimate their imminent arrest and prison sentence unless they stop contact and work with parties from whatever countries the US wishes to choose to name. Raid their houses, seize their electronics, detain them for hours in poor conditions. Lots of ways to apply pressure that doesn't even have to stand up to extensive legal scrutiny (they can keep devices and things and the people would have to sue to get them back).

The code itself is likely to exist in multiple places so if someone wanted to fork from say next week's builds for an EU build they could and there would be little the US could do to stop that but they could stop cooperation and force these developers to apply technical measures to attempt to prevent downloads from IP addresses known to belong to sanctioned countries of their choosing.

It's not like the US can slam the door and take its Linux home and China and the EU and Russia are left with nothing, they'd still have old builds and code and could develop off of those though with broken international cooperation it would be a fragmented process prone to various teething issues.