There's really little question it was intentional sabotage. Most sensible theories show Russia had means and opportunity. Including a well-equipped vessel detected near the pipelines shortly before the explosions. The motive is a bit strange, though, since it seems like Russia really wanted to be selling that oil... But it's also not hard to believe that Russia would do stupid, self-harmful things that make no sense, given the whole war in Ukraine is such a thing. Especially when there's a compelling chain of events that connect to Putin.
The Russians claim it was the US (or occasionally the UK), but they do so essentially without evidence. At least at the time of the attacks, the talk was that it would hurt NATO allies FAR more than it would hurt Russia, so it is a bit absurd. Also, when it comes to Russia, false flags are the playbook and accusations smell like confession.
There's also theories that it was a Ukrainian sapper mission. There's some evidence for establishing an opportunity, and the 'motive' of 'fuck Russia' is hardly insane. But those theories have never convincingly established means (the vessel accused was simply not capable of doing the thing and no other vessels were ever identified) and it would still not be in Ukraine's interest to hurt European allies / NATO members given their intense need to maintain that relationship. It's a bit far fetched, in my opinion. But maybe there's a logic to "this pipeline is creating an unnecessary relationship and severing it will further isolate Putin." I dunno, if the Ukrainians had wetworks logistics like that I feel like we'd see a lot more shit going wrong deep in Russia right now.
The reality is, we may never know what really happened with those bombings. It's hard to establish fact at the bottom of the sea and all investigations are almost necessary state-led ones, meaning it's easy to dismiss the evidence you don't like and stick with only the evidence that supports you. Which creates an environment full of conspiratorial speculation. And the Russians have a very effective disinformation network that captivates both the extreme left and extreme right that enjoys the chaos and uncertainty.