Should I warn my previous employer that the custom patching automation I created will break in a few months?

Nothing you (the OP) did caused this. I'd not bother. I had a very similar thing happen with some custom workflows I created at a MSP. They worked, but after I left (this was a place that laid off the Linux admins because the director of IT assumed the Windows people could run everything), the workflow broke because one of the new guys, a contractor, goofed stuff up, my backups were purged, and the Git and Wiki server I had to explain how things worked was reclaimed because the contractors hired had no clue about it, so just nuked the box and its backups, even though it was written clearly in the "bus books" I sent with managers that had recovery info, to not touch those two VMs, because it had stuff on it called "documentation", which apparently was one of the first things chucked once I was gone. A few years after I was gone, my ex-boss from that MSP called, demanded I "fix my shit". After asking him if the documentation server was online, and getting a comment that the Windows admins thought it was just UNIX crap and pitched it, I told him in no uncertain terms that I would not be working for them, not as a contractor, vendor, or consultant. I recorded the call, and when he cursed me out, I sent a recording of the call to the top brass of the MSP if this is the new norm of how they ask people to work for them. Then I had my ex-boss's boss call, begging me to fix stuff. I told him that the offshoring company that he hired that he used to replace the Linux people, the one he bragged about being "world class"? I recommended he have the "world class" offshoring company do this, because he bragged that the offshoring firm had people who were so far better than the Linux admins... that he had all the Linux admins laid off. In any case, leave that old workplace behind, unless they offer you a contract, and that contract pays for your E&O insurance.