The code below walks IPv6 tree every 60 seconds and looks into the routes with non-zero expiration time (typically, redirected routes).
For each such route it sets RTF_PROBEMTU flag at the expiration time. No other part of the kernel checks for RTF_PROBEMTU flag.
The original commit (actually, the first reference I was able to find), dates 5 Aug 1999. As far as I understood, this flag was used as a hint for the TCP to walk through it MTU table and refresh MTU values there accordingly.
There is no such functionality in the kernel anymore.
Lastly, RTF_PROBEMTUwas defined 21 years ago, 30 Jun 1999, as RTF_PROTO1.
RTF_PROTO1 is a de-facto standard indication of a route installed by a routing daemon for a last decade.
NetBSD removed it in 2011.