Does a creature that loses all abilities retain its power and toughness?

When a creature loses all abilities, what happens to its power and toughness depends on how that P/T is defined. Most creatures have their power and toughness printed as static numbers on the card. These numbers are not abilities — they are base characteristics defined in the card's text box only as numerals. Losing all abilities does not erase those printed numbers, so the creature keeps its base P/T. (CR 207.2)

However, some creatures have a characteristic-defining ability (CDA) — a special kind of static ability that defines P/T using a formula (e.g., "*/* — this creature's power and toughness are each equal to the number of cards in your hand"). CDAs are abilities, so if the creature loses all abilities, it also loses the CDA that was setting its P/T. When that happens, the game has no defined value for * and treats it as 0. (CR 207.2b, 604.3)

Note that any P/T modifications applied on top of the base (via counters, pump spells, etc.) are tracked separately by the layer system and are not themselves abilities, so those adjustments can still apply even after ability loss — but the base value question still turns on whether it was a printed numeral or a CDA. (CR 613)

Concrete example: Imagine Humility makes all creatures lose all abilities and become 1/1. A Tarmogoyf (whose P/T are set by a CDA: "*/1+*") loses its CDA. Because its P/T is no longer defined by any rule, it would be 0/1 from the CDA loss — but then Humility's "becomes 1/1" replaces that, so it ends up 1/1. Contrast with a vanilla 3/3 bear: losing all abilities leaves its printed 3/3 intact (before Humility's override).