This seriously pisses me off. We are literally watching the end of true ownership of our phones end right before our eyes, imagine if your laptop or new motherboard you purchased from MSI or whoever did the same and locked the bootloader to only allow booting official Microsoft-signed code (aka Windows only) and if you wanted to run Linux... sorry but no that's what we decided and we know better than you. Despite custom OS support being grainy in phones due to proprietary hardware and ARM chips, I really care about having the option to be able to do it (plus rooting with tools like Magisk is pretty universal across phones anyways since it lets you patch most firmware images).
It was already bad with Huawei stopping their unlock program and Google cracking down more on rooting by introducing strong integrity with their new Play Integrity API (which was an upgrade from the older SafetyNet API), basically meaning there is hardware security called the TEE (ARM TrustZone for most phones if you're interested in reading more) built into the ARM processor which "snitches? (lack of better word)" on you if the firmware booted no longer matches the manufacturer signed firmware, and causes you to fail strong integrity which means apps like bank apps can choose to deny you service (Google Wallet does this for NFC payments). There are workarounds which the custom ROM/root community still uses which mainly relies on older leaked cryptographic signing keys from the TEE being used which bypass the phone's TEE and sign the "integrity verdict" in user land to say "all is good" to Google, but Google can easily tell if these keys have been compromised since they track usage, and the storage of these keys just keeps getting better, getting as close to impossible as you can in a modern phone since to extract it would require you to quite literally de-lid the ARM chip and hope you don't break anything in the process while somehow extracting the key, in other words not feasible.
This is all great when it comes to security which Google and all manufacturers have been pushing on, but it comes at a serious cost of ownership, you cannot tell me we truly own our phones when we have literal hardware protection that, quoted right from wikipedia: "code integrity prevents code in the TEE from being replaced or modified by unauthorized entities, which *may also be the computer owner itself*". I don't know about you but a chip (and Google) that dictates what I can and cannot do with my phone doesn't sound like ownership to me.
All these recent changes and events sounds to me that Google is actively pushing and "encouraging" phone manufacturers to disable bootloader unlocking, we're constantly seeing manufacturers which were once before root and unlock friendly randomly changing their mind and quietly removing or severely limiting that feature in the background (Huawei, Xiaomi, now Samsung, etc). You have to remember these manufacturers won't back down from what Google tells them to do if it's for "security" since they're all in each other's pockets so they won't pushback without a good reason.
And if you want to use the typical excuse "allowing bootloader unlocking is unsafe", we've already proved it can work quite well while maintaining security as demonstrated by UEFI's Secure Boot which allows you to enroll custom boot keys (should you wish), while keeping some popular default keys such as Microsoft for Windows, and allowing you to lock the entire firmware config behind a password (which is stored in a security chip in modern motherboards so you can't use the old trick of removing the CMOS battery). That's more security than any regular citizen might need.
This TEE thing is all about control. Google and manufacturers don't like people installing custom firmware or rooting because then they can't keep you in their ecosystem to keep taking your data and hoping you eventually buy something from them. Some app developers also think this locking down of phones is great in order to protect their app against abuse than actually investing in good backend security which I just find to be hilarious.
I hope some laws get passed to protect us from the 1984 book that society is starting to become thanks to the government and corporate conglomerates themselves, although I sadly find that to be unlikely.