I'd posit that Linux having a hard policy of "do not break user space" has always just been "mostly true", especially around architecture specific stuff, and stuff tied to corporations.

I bring this up only because recently, I made it my mission to get IBM's "PowerVM Lx86" dynamic translation system running on a POWER9 system running modern Debian Linux.

This required a lot of hackery I won't go into here, but it revealed two things to me:

1. The lx86 system depended on an undocumented (??) pseudo-syscall to switch between big and little endian modes. This was "syscall 0x1EBE", which was implemented in the exception handler for PowerPC. In other words, it wasn't a real syscall, and tools like strace do NOT capture/log it. It was a conditional branch in the assembly entry point for the process exception handler which switches endianness and then returns. Quicker than a "real" syscall. Also, long gone in the Linux kernel, replaced with syscall switch_endian, hex 0x16B. Adding this in wasn't too hard, but it'd sure as heck never make it upstream again ;)

2. A lot of other Linux calls have had bits added which break old applications that are "rigidly coded". For example, netlink has had extra data stuffed into various structures, which code like lx86's translator panics on. To get network stuff running, even with an antique x86 userland, required removing a bunch of stuff from the kernel. Not disabling it via configuration, but deleting code.

All this to say, there is a precedent for breaking the Linux user facing ABI for hardware features like this. I'm not saying that's a good thing, but it is a thing.