Does 5e no longer have the specific spell Stone to Flesh, to deal with this?
I don't think it does, Greater Restoration is normally used for this.
In any case, were it me I'd rule two things: one, a petrified "creature" is now made of solid stone; and two, polymorphing it would only work if you can somehow otherwise polymorph a piece of rock.
You can polymorph any creature, including one made of rock (or fire, or wood, or glass ....). A petrified stone PC, a wooden twig blight, a stone golum, a fire elemental- what they are made of is not relevant to the spell and they can all be polymorphed.
However a stone creature like a Stone Golem is not petrified before the polymorph, while the PC is, so IMO the PC stays petrified after you transform him into a mouse (a small stone mouse), while the stone golem becomes a flesh, blood and fur mouse. The exception would be if the creature you polymorph him to is immune to petrified, in that case the PC would not be petrified while polymorphed, but I don't think that applies to any beasts that would be available with the Polymorph spell.
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