Resolution: Works As Intended
Opening https://sessionserver.mojang.com in your browser returns
<h2>Our services aren't available right now</h2><p>We're working to restore all services as soon as possible. Please check back soon.</p>0UnuEYwAAAABQnNS41COKTpK/CPf2rSL3RFVTMzBFREdFMDYxMwBFZGdl
for quite some time already. I already read that this server is no longer in use. This causes confusion however, as I was setting up a modpack-server and had trouble with the authentication servers. In the logs I could see that it was trying to call this server so I thought Microsoft had an issue here and it wasn't my fault. To prevent further confusion I would suggest changing the message to something like "This service is deprecated since XYZ. Please use XYZ instead. Relevant documentation: XYZ"