Well as a veteran player who has played league and watched esport since the end of season 2, I personally think that even though the game has gone down hill since the end of season 5, it's still playable until the release of patch 8.4, when they redesigned all AP items, and the game has been extremely unbalance since that point.
I will break season 8 into 3 periods, plus the season 9 until now, to create a 4 periods of time, as it's pretty unfair to compare season 8 and season 9 when it's just patch 9.4 now. Note:
1/ 8.4 until 8.5: After AP items redesign and prior to Conqueror release. (Less AP build paths and the game is more bot-lane focusing)
2/8.6 until 8.10: After the release of Conqueror rune and prior to the AD items redesign. (Which started the Bruisers Era).
3/8.11 until 8.24b: After the AD items redesign and before Akali got nerfed. (Botlane AP carriers, and Bruisers and Akali era: Akali, Irelia, Urgot, etc...)
4/9.1 until 9.4: Current meta. (Tp, ignite got nerfed, the raise of Lucian and Vayne, extremely high shutdown gold bonus, early gold inflation because of the Turret plates, etc...)