The ESP8266 Core 2.6.3 has been released 2 days ago.
The transition from core 2.6.2.to 2.6.3 is very smooth. No issue so far. The code size, depending on the project, is up to 700 bytes larger.
The problem of
esp8266com/esp8266/cores/esp8266/core_esp8266_features.h has been fixed.
These important bugs have also been fixed:
The 'inline' attribute on some of the routines seemed to override the 'ICACHE_RAM_ATTR' attribute, and some of the slave ISR routines were ending up in Flash, or were split; part was in Flash, part in IRAM. This caused exceptions when I2C slave events happened during OTA download.
UDP Crash with versions above 2.5.2(https://github.com/esp8266/Arduino/pull/6895)
This commit avoids OOMs (Out Of Memory) on an udp corner case where a delay() in the main loop would allow memory filling.