GURPS Low-Tech (4e) makes mention of hobnails on p98 (text box), but while it does mention the improved footing in general, and reduced Stealth on hard surfaces, it does not mention the rather important point that they also
reduce footing on hard, relatively-smooth surfaces, as Lindybeige notes in
this video, helpfully citing an example from Josephus's
The Jewish War (Book VI, Chapter I). I'm reasonably sure that other examples can be found, but I have not seen the problem dealt with in GURPS.
Of course, if you're doing Yrth or another non-historical game, the GM can just say that this culture nails/glues strips or discs of leather to the bottom of their shoes/sandals, which does a 'good enough' job of imitating the fancy rubber soles we use (if they don't have rubber, themselves). If you
are doing a historical game, though, and you want to go that far in simulating historical accuracy, an appropriate rule for this could help.
Thoughts?