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ESP8266 has been quite popular. After years of community efforts, it was the de-facto platform for WiFi-enabled projects. Espressif, the chip vendor of ESP8266, has been releasing ESP32-based products. ESP32C3, supposed to be a successor of ESP8266, was introduced into market in 2020. Should we switch to it? Or ESP32? I would say it is time to say goodbye to ESP8266.
Summary
Use ESP8266 where you don’t need reliability or security. Use ESP32 and its variants where you need reliability and security. Use ESP32C3 or ESP32 where possible. If you are a commercial user of ESP8266, good luck.
esp-idf and ESP8266 RTOS SDK
esp-idf, Espressif’s official IoT Development Framework, is one of the greatest SDKs by vendors. It is a fully open-sourced, truly community-driven SDK for ESP32 series. Its documentation is well-written, maintained, and up-to-date. The SDK supports all the ESP32 series.
ESP8266 RTOS SDK, the official SDK for ESP8266, is not maintained as much as esp-idf is. The documentation is sparse, and out-dated. Its README says they have a plan to integrate the SDK into esp-idf, but the statement has been there for many years without progress. While the SDK has incorporated many features and compatibilities from esp-idf, but not quite compatible (SPI driver, for example. I ported many drivers from ESP32 to ESP8266, but not SPI-based ones).
There is another official SDK, ESP8266 Non-OS SDK. This SDK does not include FreeRTOS, and Arduino ESP8266 is built on top of it. However, this SDK is now obsolete, and Espressif will not maintain it. It is unlikely for Arduino ESP8266 to support ESP8266 RTOS SDK (see the discussion in Issue #5790). That means ESP8266 Non-OS SDK, and Arduino ESP8266 as well, will not have a bright future.
Instability
I use many ESP8266 at Makers. Most of them are for simple purposes, such as ambient sensors or relays for non-critical machines. The problem is, these ESP8266 devices reboot — actually crash — sometimes. The crash is not very reproducible; even an identical firmware crashes every 10 minutes, or keeps running for several weeks. There are many similar reports in esphome repository. One developer guessed the cause might be corrupted flash memory, or a bug deep inside of the chip. For ambient sensors, reboot is not critical if it reboots and becomes online again. But for relays that control critical machines, such as 3D printers, the impact is much bigger. If reliability is important, I will not use ESP8266.
Security
ESP8266 does not support hardware encryption, or tamper protection. That means you can dump the firmware, including secrets, from the flash memory. It is impossible to secure the chip. When your secrets on the device is just WiFi SSID and password, that might be acceptable, assuming the device has a certain level of physical security. Otherwise, ESP32 is the only answer.
Encryption at software layer, i.e. HTTPS, requires lots of memory, and some chips do not have enough memory, i.e. esp01. Recent ESP01 has more memory, but you might give up OTA, or Over the Air update.
Alternatives
As officially stated, ESP32C3 is the direct descendant of ESP8266. The price cannot compete with ESP8266 yet, but cheaper than ESP32. esp-idf fully supports it, and quite usable from my experiments. Less GPIO pins than ESP32, but more than ESP8266.
ESP32-S3-MINI is another alternative, but not very commonly available.
ESP32 is also an alternative, but its power requirement is much higher than ESP8266.
Where to use ESP8266
I would choose ESP8266 for ambient sensors, and other non-critical purposes only. I might use it as an alternative for AVR series because it has more memory (but less GPIOs, less ADC, and power hungry). Light control with PIR motion sensor? Maybe. Relays? No. I have several Sonoff relays, but will use them for lighting systems or fans where a short disruption is acceptable. Mobile use cases are good for ESP8266, where physical security is guaranteed. Deep-sleeping devices are another use case for ESP8266, where the random crash issue I mentioned is not a problem.
Conclusion
Although it is not EoLed, ESP8266 is approaching to its end of life. Use modern ESP32 series with esp-idf. It was a great product that made ESP32 possible. We loved it. Thank you, ESP8266.