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

Power and toughness are distinct characteristics of a creature, separate from its abilities. According to CR 207.1, a card's power and toughness are defined by the numbers printed in its lower-right corner (or set by continuous effects). Abilities are a different characteristic entirely, governed by CR 113.1. Stripping abilities does not touch power/toughness unless the effect explicitly says so.

When an effect says a creature 'loses all abilities,' it removes keyword abilities, activated abilities, triggered abilities, and static abilities (CR 113.1). It does not alter the creature's base power and toughness or any continuous effects that are already setting or modifying those values.

It's worth noting that if a creature's power or toughness is defined by a characteristic-defining ability (CDA) — for example, a creature whose toughness is '*' equal to the number of cards in your hand — and that CDA is removed, then the value it defined reverts to the last set value or 0, per CR 208.2a. This is the one condition where losing abilities can indirectly affect power/toughness.

Example: Your opponent controls a 5/5 Ruric Thar, the Unbowed. You cast Sudden Spoiling, which gives it -0/-0 and causes it to lose all abilities until end of turn. Ruric Thar becomes a 5/5 creature with no abilities — still a 5/5, still able to attack and block normally, just stripped of all its text.