>>105897313For most users, the domain will stop resolving, but Russian ISPs could have their own DNS that continues to support them, in which case users in Russia who use their ISP's DNS would still be able to access them.
My guess is that when the 2030 deadline approaches, there will be a five year extension, and in 2035, ICANN will delete .su, leaving it up to DNS providers to decide if they want to keep supporting it.
By then most sites should have moved to one of the other domains. The real risk will be that some unknown daemon somewhere is running an important background task everyone has forgotten about, with a hard coded reference to a .su domain. If it no longer can get the IP address for that domain, the task will fail and something will stop working for no apparent reason. Maybe it will be a crosswalk sign. Maybe it will be a radio beacon. Maybe it will be the music being piped into a rural market in Siberia.