OK, a quick search of D&D beyond shows these things rounding down:
- Ritual Caster feat in the PHB
- Shield Guardian monster stat block from the MM
- Cleric variant feature from Tasha's
- Living Armor from Eberron
DDB will only return 35 results, and that's just for sources I have unlocked and only 1st-party sources. In fact my original query was prompted after I saw a particularly elegant mechanic in a homebrewed class that was marred by a functional-yet-ugly "(rounded up)."
However... There are also like a bazillion results for "(rounded down)" which kinda shows that the global round-down rule isn't even very effective if they feel the need to re-specify it all over the place!
I don't want to blow this out of proportion and seem like I'm crying and losing sleep over this "always round down" rule. It's just that I've lived with the rule for so long and I never really questioned it, and now that I do, it seems vestigial.