Gatekeeping. The only truly consistent definition of “Leftist” 😀

Yes “leftist” is relative to the overton window in your current time and space, but at the same time this argument has been used to co-opt and dilute movements.

So yeah the democrats for example can call themselves leftists, but no real leftist will take them seriously.

Additionally you can always use more precise terms to avoid this. Like anarcho-syndicalist, Marxism-Leninist, etc.