If you can track down a copy of the IBM XT or AT technical reference manuals, you will find a complete listing of the BIOS. Not only is it a substantial piece of assembly code, but you might even learn a bit about how a PC starts up. Ok, maybe not the latest EFI ones.
Look into the demo scene and their multiple groups and events for hyperoptimised assembly on sometimes exotics platforms.
Alternatively, reverse engineering and vulnerability research is, while less focused on writing assembly and more on understanding it, another area where you'll find competent people.
Old, constrained 8-bit games platforms are full of fun tricks, though most of them are not really applicable to modern x86 assembler. There are commented Atari 2600 disassemblies available. My favorite to just read top to bottom is Pitfall. Relatedly, you can find "demo scene" content that also really pushes the limits of these machines, though most of these tricks are more related to bending/pushing the antique video hardware than slick assembly techniques per-se.
Go read @a2_4am's reverse engineering of old Apple ][ copy protection code. The code itself is amazingly tricky and the work to decipher it even more so.
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Alternatively, reverse engineering and vulnerability research is, while less focused on writing assembly and more on understanding it, another area where you'll find competent people.