NodeMCU formerly provided 7 static timers, numbered 0-6, which could be used instead of OO API timers initiated with tmr.create(). After a long period of deprecation, these were removed in 2019 Q1.
Hence, if you're still using firmware with the index-based timers you're advised to upgrade.
- regularly check https://github.com/nodemcu/nodemcu-firmware/releases
- set the GitHub repository watch level (menu at the top) to "Releases only"
- follow the community on https://www.facebook.com/NodeMCU/