The whole point of saying something is a Main or Flexsupport is only to help creating a rister, so the 2 Support players are able to play nearly every good combo, thats why outside of organised teams nobody uses the term this way
Just because you dont know the definition it doesnt mean there isnt one. You make a declation of the roles because of the names.
Flex support was called flex supporz because it wasnt required back then. In the past, expecially during Dive, we had many viable 3 DPS comps and Flex Support was a role wich needed to know other Hereos too (They tended to play Widow in many 3dps conps for example)
Mainsupport is called mainsupport because you always need one, you cant play a Flexsupport without a Mainsupport. In every triple Dps comp for example you used Mercy or Lucio
After 2-2-2 roleloxk the names of the roles just keot getting used because you are used to it
Mainsupport is just a Support who can get played without a 2. Support