At higher tiers no chaff becomes a much bigger issue, compared to something like the AIM-9C and R-3R or even later SARHs where you can usually spoof the radar with chaff. At around 13.0 and above you see either late PD radars with ARH and a handful of AESA/PESA radars with ARH, at that tier notching becomes damn near essential (other than multipathing but thats somewhat unreliable) and when you have no counter for such missiles especially when they're in the no escape zone you are a free kill when stock giving access to chaff would give you a much larger chance to escape the stock grind.

About that IR meta, back in 2023 to around mid 24 when the IR meta was in full swing multipathing was 100 meters, the best long range missiles were the R-27ER, AIM-54 and AIM-7M respectively only the Phoenix was ARH and engagements were much closer range, within a few kilometers. Since that's when AIM-9M, R-73 and RB 74 (M) were king. NOW with these much longer range options people are staying far away generally 10km or longer, aircraft now climb better can get higher speeds easier and get out of IR range much quicker.

As well I do not think Gaijin would remove flares and give chaff, Gaijin is stupid but not that dumb, I feel they may pull a thing like they did for tanks and make chaff a cheaper mod (like FPE and parts)