No, Mysterium isn't, but Mullvad just partnered with Obscura to create an onion routing like service. I asked Obscura if it worked with Tailscale here:

X (formerly Twitter)

Mike Hodl Castle (@mdhardcastle) on X

Hey @obscuravpn can I use you if I'm using @mullvadnet inside @Tailscale ?

Obscura

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Mullvad VPN

Mullvad has partnered with Obscura VPN | Mullvad VPN

Today we are announcing a partnership with Obscura VPN, a newly launched two-party VPN service that uses our WireGuard VPN servers as its “exit h...