That blog post is very vague. "Undesirable content" could mean anything depending on the jurisdiction. Also they did not state which specific laws were being broken, or whether there was a court order to remove PF, which seems like pretty important details to leave out if you are trying to be transparent. (unless they are under some kind of gag order due to an ongoing investigation, in which case, yikes.)
If they were not legally compelled, then the only other explanation is Mullvad made a voluntary decision to remove PF, which means they are not being neutral and have chosen to discriminate against a technology widely used by p2p users (which the anti-p2p overlords definitely will be celebrating). It might have been for their own convenience, or for personal moral reasons, which is entirely within their rights if they choose to do so, but it is an odd stance for a VPN provider to take, and indicates a lack of willingness to fight on behalf of their users.
It's also worth noting that in an earlier blog post, Mullvad claimed the recent law enforcement visit was their "first in 14 years", and that it was due to a "blackmail" case in Germany from 2021. For a service that is supposedly abused in the ways you describe, I find it hard to believe that their "first visit" would be over something as mundane as an old blackmail case. https://mullvad.net/en/blog/2023/5/2/update-the-swedish-authorities-answered-our-protocol-request/
If the climate has gotten so hostile that VPNs can now be legally compelled to remove essential features like port forwarding (there is thus far no concrete evidence of that being the case, only speculation), then all VPNs are in trouble, and we are truly in danger of losing the free and open internet as we know it.