how exactly do leavers affect pbsr?
Check this post for details:
When someone on your team leaves, the enemy team has less to distract them during fights. That means you’re more likely to die, and less likely to be able to do your job effectively.
If the leaver is a tank, less space will be created, DPS can’t properly do their job, and supports die quicker, so everyone else will too.
If the leaver is a support, your tanks can’t be kept up as well, and DPS can’t properly do their job, so everyone dies quicker.
If the leaver is DPS, you’re not outputting enough damage to secure final blows, so in a team fight you die quicker.
Being down a player means less final blows, so less eliminations in general, which means lower stats, which means less SR.