The Wikipedia article for Theseus ends its introduction with this:
As the subject of myth, the existence of Theseus as a real person has
not been proven, but scholars believe that he may have been alive
during the Late Bronze Age possibly as a king in the 8th or 9th
century BC.
This assertion is based on Classical Mythology Tenth Edition. Quoting the relevant parts:
History and Legend. Theseus of all the legendary heroes has the strongest claims to being a real person. As stated above, he was for
Plutarch a historical figure and he very likely was one of the kings
of Athens perhaps in the ninth or eighth century B.C. But serious
historical and chronological problems arise when we try to understand
how he appears as the great conqueror of the legendary Minotaur and a
king of Athens in the earlier Mycenaean Age and also a later king of
Athens who has more serious claims to reality. Were there two
Theseuses or only one, around whom all the stories clustered? Some
scholars question the traditional dates established for the Dark Age
that descended upon Greece after the fall of the Mycenaean kingdoms,
ca. 1100–800 B.C.; on the basis of comparative studies, they would
eliminate this Dark Age altogether or at least place the chronology of
the legendary Mycenaean kings of Athens much closer in time to the
chronology of the later historical monarchy, thus making one
historical Theseus more comprehensible.
The excavations of Sir Arthur Evans in Crete seem to confirm details
of Minoan-Mycenaean and Athenian saga. For one thing, the elaborate
palace at Cnossus with its complexity of levels and maze of rooms does
suggest a labyrinth. For the history and archaeology of the Minoan and
Mycenaean period, see MLS, Chapter 2.
Morford, Mark; Lenardon, Robert J.; Sham, Michael. "Classical Mythology Tenth Edition". Oxford University Press. Oxford University Press.
Worth mentioning too the idea of Euhemerism:
an approach to the interpretation of mythology in which mythological
accounts are presumed to have originated from real historical events
or personages. Euhemerism supposes that historical accounts become
myths as they are exaggerated in the retelling, accumulating
elaborations and alterations that reflect cultural mores. It was named
for the Greek mythographer
Euhemerus, who lived in the
late 4th century BC. In the more recent literature of myth, such as
Bulfinch's Mythology, euhemerism is termed the "historical theory" of
mythology.
