Sure it is. mw42815 wrote about a 5-6 year old, limited problem (firmware modified to make it impossible to use AHCI instead of RAID) affecting a few Lenovo models sold in the Microsoft Store, but not others, and headlined the thread ("Miicrosoft making it impossible to install Linux") in a way that suggests that the problem is both current and widespread.
SOLUSfiddler Linux systems can boot using signatures on an activated Secure Boot UEFI system. But Microsoft has published an update that withdrew a lot of those signatures so that Linux systems aren't able to use them any more, resulting in the fact that they cannot boot on machines running Windows.
A bit of an overstatement, to say the least. Microsoft requires any computer running Windows 11 to be Secure Boot capable, but Microsoft does not require that Secure Boot be enabled. Windows 11 computers also come with Bitlocker enabled in many/most cases. Both, along with the TPM-capable requirement imposed by Microsoft in connection with Windows 11, have been reported widely for well over a year now.
None of this means that Linux "cannot boot on machines running Windows", as you suggest.
Secure Boot can be disabled and Windows 11 will run with Secure Boot disabled. Bitlocker can be disabled, and Windows 11 will run with Bitlocker disabled. Disabling Secure Boot and/or Bitlocker (if enabled by the OEM manufacturing the computer, as is almost always the case with respect to Secure Boot and is becoming more common with respect to Bitlocker) requires user action, but we have been dealing with that for years in the Linux world. At least I have, because the Dell Optiplex and Latitude models that I use have both Secure Boot and Bitlocker enabled OTB and I have to disable both in order to run Solus.
The c't article reports a single change: Microsoft has "blacklisted" many/most Secure Boot shims, including older shims from maintstream distros like Ubuntu, and required new shims that comply with current Microsoft security standards. That isn't unreasonable, given the enterprise-level and enterprise-driven security measures taken by Microsoft with respect to Windows 11, and, as is the case with the Secure Boot, Bitlocker and TPM, Microsoft announced the change well in advance, and third-party vendors (e.g. Ubuntu) have had more than enough time to develop new shims and recertify, as Ubuntu did with 22.04.
None of this is ideal for Linux developers/maintainers, of course, and is inconvenient for users. But it isn't the end of the world, either, or evidence that "Microsoft making it impossible to install Linux", as the title of this thread hypes. Solus has never been compatible with Secure Boot, requires Secure Boot to be disabled in order to install, and runs fine alongside Windows 11.
I think that it would be inappropriate for Microsoft to make "it impossible to install Linux", but I think that it is equally inappropriate to demand that Microsoft ignore Microsoft considers appropriate security measures in order to accommodate Linux.
