As for your examples, I think both of those files (interrupts and slabinfo) pre-date /sys so they cannot have been put in /proc to avoid /sys rules.
Also I cannot see any atomic-consistency issues that might arise when accessing any of this information. In fact the contents of /proc/slabinfo appear to also exist in /sys/kernel/slab/*/* which does seem to suggest independent access to the values is OK.
If there are genuine cases where consistency is needed it would be really helpful to have them documented so we can work towards making sysfs receptive to such need.
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