They're only killing the crappy store/UWP version that nobody used anyway and only caused confusion. The normal OneNote bundled in Office isn't going anywhere as far as I know.
That said, I've moved a lot of my note taking to Obsidian. It's not a perfect replacement but it's a fantastic markdown editor and now I use both for different use cases.
To add, for Linux kernels, the maintainer use a shim EFI package with the distro's keys (e.g., Canonical's keys for Ubuntu) which loads the maintainer-signed kernel. And Microsoft signs the shim to keep the chain intact.
Nothing you said is wrong, in fact it's all good advice. But none of what you listed implicitly provides protection against ransomware either.
For that you need backups that are immutable. That is, even you as the admin cannot alter, encrypt, or delete them because your threat model should assume full admin account compromise. There are several onprem solutions for it and most of the cloud providers offer immutable storage now too.
And at the very least, remove AD SSO from your backup software admin portals (and hypervisors); make your admins use a password safe.
"To read the purported PDF document, victims are persuaded to click a URL containing a list of steps to register their Windows system. The registration link urges them to launch PowerShell as an administrator and copy/paste the displayed code snippet into the terminal, and execute it."
Yep that's how I have Syncthing set up. All global and local discovery disabled, no firewall ports open on the clients, no broadcasting, no relay servers. Just syncing through a central server which maintains versioning and where the backups run. Works like a charm.
As another poster mentioned, QubesOS with anti evil maid will work, but that's the defense against state actors too and is overkill for this threat model.
BitLocker or any FDE using SecureBoot and PCR 7 will be sufficient for this (with Linux you also need PCRs 8+9 to protect against grub and initramfs attacks). Even if they can replace something in the boot chain with something trusted, it'll change PCR 7 and you'd be prompted to unlock with a recovery key (don't blindly enter it without verifying the boot chain and knowing why you're being prompted).
With Secure Boot alone, the malicious bootloader would still need to be trusted (something like BlackLotus).
Also make sure you have a strong BIOS password and disable boot from USB, PXE, and anything else that isn't the specific EFI bootloader used by your OS(es).
Not that it's my first recommendation for security reasons, and I would never do this in prod, but you can just add the self-signed cert to the local trusted root CA store and it should work fine. No reg changes needed.
If you do this, put it in the store of the user running the client, not LocalMachine. Then you just need to make sure you connect as something in the cert's SAN list. An IP might work (don't know since I never try to put IPs in the SAN list), but just use a hosts entry if you can't modify local DNS.
Edit: after reading the full OP post (sorry), I don't think it's necessarily the self-signed cert. If the browser is connecting with https:// and presenting a basic auth prompt, then https is working. It almost sounds like there is a 301/302 redirect back to http after login. Check the Network tab of the browser's dev pane (F12) to see what is going on.
Microsoft uses TPM PCRs 7+11 for BitLocker which is more secure than the Linux implementations mentioned in the article.
PCR 7 is the Secure Boot measurement which means it can't be unlocked unless every signed boot component has not been tampered with up to the point of unlock by the EFI bootloader. PCR 11 is simply flipped from a 0 to a 1 by the bootloader to protect the keys from being extracted in user land from an already booted system.
The article is correct that most Linux implementations blindly following these kinds of "guides" are not secure. Without additional PCRs, specifically 8 and 9 measuring the grub commands (no single-user bypass) and initrd (which is usually on an unencrypted partition), it is trivial to bypass. But the downside of using these additional PCRs is that you need to manually unlock with a LUKS2 password and reseal the keys in TPM whenever the kernel and or initrd updates.
Of course to be really secure, you want to require a PIN in addition to TPM to unlock the disk under any OS. But Microsoft's TPM-only implementation is fairly secure with only a few advanced vulnerabilities such as LogoFAIL and cold boot attacks.
They're only killing the crappy store/UWP version that nobody used anyway and only caused confusion. The normal OneNote bundled in Office isn't going anywhere as far as I know.
That said, I've moved a lot of my note taking to Obsidian. It's not a perfect replacement but it's a fantastic markdown editor and now I use both for different use cases.