I beg to differ about XMS. That particular technology may have only been relevant for a decade, but the idea of using a harder-to-access storage to supplement cheap-but-limited storage is everywhere. L1 and L2 cache, data warehouses, cloud storage, and so on. I value learning about that abstraction early on. I’d agree it’s not singularly career changing, but I don’t think knowing any one technology in the software industry is.
XMS wasnt a supplement, and its behaviour wasnt analogous to a cache. It was a 32 bit wrapper that would allow 16 or 32 bit* dos applications to access all "extra" memory that 32 bit systems usually got installed. The access was commonly provided via emulating the EMS mechanism - a 64k or 128k block to be paged within the fist 2^19 bytes, or by providing a 32 bit address for a usable block.
*32 bit and protected mode (ring3) support was provided usually via DPMI, an interrupt service managed by a TSR (daemon), or VCPI, a more complex privileged system. Some DOS extenders could also provide this functionality for their own binaries, notably the very popular DOS4GW.EXE, the Watcom dos extender. Yes, the name of the file reflects the physical address limit of i386 systems, 4GB. (note that when working with linear addressing, the max addressable range is quite superior due to the paging mechanism).
