I know that @Fishpo31 was there. Baseball was our first big sport and until 1970 or so the players had no rights and the owner's had the reserve clause and the pay was horrible. Ball Four takes a good look at this in the 60's with the Yankees who had the biggest stars. And only the biggest stars got close to 6 figures
Marvin Miller changed everything. Now the strongest union by far with salaries that don't seem to match the waning interest in baseball but there they are
The NBA is second for the union. Both have guaranteed contracts.
Then America's Game the NLF with no guaranteed contracts are finally being dragged to up the up front guaranteed money. The most dangerous sport with the shortest lifespan pays the worst. Go figure