Libevent 2.1.6-beta was released in July this year.
The current stable build (2.0.22) was released in Jan 2015, the same time as the previous beta.

Not using 2.1.x is affecting atleast

• [WIP] rest: Stream entire utxo set [WIP] rest: Stream entire utxo set #7759
• bitcoin-cli: More detailed error reporting bitcoin-cli: More detailed error reporting #8722
• Compatibility logic in bitcoin-cli.cpp

There are also compatibility issues with 2.0.22 and recent mingw-w64 gcc versions, see #8730

Optimistically tagged this as something for 0.14.0