Earlier editions of D&D had rules for how powerful deities were if you were ever to fight one, including class levels, statblocks and special powers.
Does D&D 5th edition have any stats on deities, or rules on how powerful they are?
Yes - there is one.
Rise of Tiamat states on p. 87 "Tiamat is a god" and gives statistics for her in the Appendix.
She is given a CR of 30 and the book says: "If she manifests through the portal at full strength, she can demolish multiple 15th-level parties with ease."
Depending on how broad your definition of deities is, you might include the demon lords from Mordenkainen's Tome of Foes. Although their worshipers are comprised of pretty much only the most insane cults, they are worshiped, grant spells, abilities, etc. to their followers.
Their CR is in the 22+ range, but that doesn't include their Lair, which is where you'd need to kill them for them to stay dead.