Inspired by this question: Multi purpose Arcane Focus?
The premise of the other question is about using the Rod of the Pact Keeper as a focus, and although it implies it can be used as a focus indirectly, I cannot see in the description of the magic item (DMG, pg. 197) anywhere that says it can be used as a focus, nor in any generic text about rods (DMG, pg. 139).
Rod of the Pact Keeper
Rod, uncommon (+1), rare (+2), or very rare (+3) (requires attunement by a warlock)
While holding this rod, you gain a bonus to spell attack rolls and to the saving throw DC of your warlock spells. The bonus is determined by the rod’s rarity.
In addition, you can regain one warlock spell slot as an action while holding the rod. You can’t use this property again until you finish a long rest.
Given that it isn’t explicitly said in the description, I assume it must be possible for all rods to be used as foci. RAW, can any rod be used as a focus?