Extra turns and crit rate are indeed great to compare.
Crit rate is a chance, if it procs, it procs, increase beyond 100% chance has no effect.
Gear or skill give a chance for an extra turn. If it procs, it procs. Increase beyond 100% has no effect.
The extra turn the main price of the lottery. After you allready won the main price, you can't say "but my second ticket has a 18% chance for the main price. That is not true anymore! 😭 Buy a second main price for your lottery, and if my second ticket procs, give me both".
Athel can still get 3 turns in a row. If you use her A3, you get an extra turn with a chance of 100%. In that extra turn, relentless set has an 18% chance to proc, like allways (new turn, new lottery). The only thing that does not happen is, that a proc from relentless in turn 1 is saved and automatically gives another extra turn in addition.
It was not even a nerf (nerf = changing an initially wanted game mechanic, because it turned out as to strong), but a bufgfix (bugfix = changing an unintentionally game mechanic that was never wanted by the developers). In the Patch notes of 1.14 the change in relentless is listed under bugfix. Nobody ever wanted stored extra turns.
Patch notes
To clarify,
Champions can still get multiple turns via the Relentless Set. However,
where before the engine calculated both extra turns from Skill usage and Relentless
proccing at the same time, 'saving' those turns to be used immediately
after the first one, now it only calculates the extra turn from the
Relentless Set or the Champion's
Skill at a single given instance. Once that extra turn has been
obtained, the engine no longer attempts to calculate additional extra
turns.
But if the Champion gets an extra turn from their Relentless Set and then uses their Skill for another extra turn, it will work exactly as intended and vice versa.