For 2021 tax filing
someone with w2 income as well as some 1099:
If some ability to work that 'side job' was affected by the relevant reasons (sick/quarantine/childcare etc.) can file FFRCA paid 'sick leave' against the SE taxes being charged on that 1099?
Yes
Does it require equivalent amount of sick leave taken on W2 ?
No, although if FFCRA is taken across multiple jobs and/or W2 + 1099, then the individual is capped at the $12k maximum across all source of income/employers, and not $12k per employer/income source.
What documentation of leave is required for self employed filing?
No official docs, but if the reason is for an order of quarantine, document the name of the healthcare provider. If the reason is for childcare, document the name of the child and the name of the school. Keep these for a period of 4 years in case of audit.
(thanks for bringing the question to my attention- I saw it a few days back and was going to check up on some details, and it slipped my mind).